Y'know, a lot of corporations are also listed as being led by nobles - making them fiefdoms in their own right, and making the nobles eligible for their own 'Mech forces... which are then vulnerable to being called up.
Oh goodie, this topic! It's got to be one of my favorite subjects about which to argue.
To recap my views I've expressed in threads past in short form, I think there are "a lot" of mechs and mechwarriors outside the Great House armies. It's impossible to give an exact ratio, but my gut (and my headcanon) puts it at for every 1 mech in a Great House Army, there are 3-5 more in corporate, petty lord, pirate, or merc service.
I... disagree.
1) Mechs are prestige units. BattleTech is centrally focused on the BattleMech in a meta sense (for example, that's why WarShips had to be knocked down several pegs) but I'd argue that's also true in an in-universe sense. Materials as far back as MW1 talk about how a practical measure of a Noble's power was not so much the size of his fief but a count of how many Mechwarriors he had sworn to his service. The rich and powerful may not need BattleMechs, but having them is a measure of success and the simple sociology of "Keeping up with the Joneses" ensures there's a lot of BattleMechs around in Nobles' private armies. The same phenomenon extends to Corporate rivalries.. if Coke deploys a regiment of BattleMechs to secure their facilities (and bully their rivals) you know the Pepsi Regiment won't be far behind.
I think you're missing the main point of feudalism:
Power.
Just as (normal) feudal kings kept careful track of how many men their dukes had under arms, dukes made sure their earls and barons weren't stockpiling suspicious amounts of weapons and horses, earls and barons made sure their knights adhered properly to household limitations, and knights made sure the yeoman and peasants didn't have old swords or proper armor, so to did the Inner Sphere function on the theory of limiting just exactly what kind of forces your underlings had, lest they rise up and take your position... and be
right in doing so, because if you're too weak to protect yourself, you're too weak to protect your underlings!
It isn't about how much materiel a given Successor State COULD manufacture in a pinch during the Succession Wars, it was about how much materiel a Lord
trusted his nobles to have - balanced by their need to make sure their realm could defend itself, and without accumulating so much strength to themselves that they'd be practically inciting rebellion out of fear. But it's a balancing act ALL the Lords had to do, and so the balance held... at least until the Clans invaded.
2) There's not enough mechs in a House Army to cover its Empire. This one is the closes that comes to a mathematical argument... Take a look at any sourcebook ever that codifies a Great House Army's mech deployment. Notice how few worlds actually get a mech garrison compared to how many don't have any Great House mech garrison at all. Very roughly, it's about 3 or 4 worlds for every 1 world that does get a listed mech garrison. Now granted, those worlds may indeed have a Great Army Conventional garrison that happens to not have any mechs. But remember point 1. Mechs are primary, and politics are local. If Planet X didn't get a mech garrison from the interstellar empire, it will generate its own, because in Planet X's point of view it's the most important planet of all and gosh darn it, it deserves mechs! It's militia will have them, and sure as tootin' its petty nobles will have them.
And once again, it's about what the lord lays down, and what the people accept from said lord. If Duke Sandoval wants his regiments concentrated on Galtor III, Quentin, and Bergman's Planet, poor little Count Crapeater of Sauk City has to grin, send the battalion that's nominally his, and eat the crap rolling downhill on his head.
3) Segueing from point 2 is the Piracy argument. Pirates have mechs. House army mechs are not on most planets. What's supposed to stop Mech-armed pirates on most planets, tanks and infantry? Yeah, that's not gonna fly. Most planets have a very practical incentive to arm up some mechs in the threat of Piracy. Even those planets that DO rate a Great House Army mech garrison have reason for indiginous mech forces because of the threat of Piracy... the House Mech garrison is more focused on the enemy House Mech forces a jump or two away than ensuring Pirates are prevented from looting the planet's megamarts or making off with slaves from the planet's remote communities.
Why didn't feudal knights teach their peasants how to build hedgehog fences, simple spears, and basic fighting to defend themselves against bandits? The same reason that Successor Lords don't give 'Mechs out to anyone that isn't directly sworn to them: it's a short step from "Defending what's mine against bandits!" to "Defending what's mine against the lords who ain't never worked a day in their lives!"
It's about
power, and making sure only certain people have it.
4) The TRO Treadmill argument. The necessity of publishing TRO after TRO is completely meta, with any possible in-universe explanation is contrived. However, the meta fact that there are a steady stream of TRO after TROs means that there's a flurry of mech production going on in the 31st century... there has to be or else there wouldn't be new mech classes (and new mech variants) to discuss in said TROs. We don't know numbers of individual mech classes or variants, but we *do* have a basement on the number of classes/variants... and that number is those presented in the TROs. And that's only the minimum number, as there can always be more canonical classes/variants added/retconned in at a date after the TRO is published! Couple that number with another rarity of a known number: the size of the mech fleets deployed for battle by the various Great House Armies. With all these new (kinds of) mechs coming into service, without the House Army numbers blooming, you come up with several explanations.
But didn't the militaries expand? Circa 3025, the Draconis March 44 regiments; circa 3050, it had 50 - and paper strength, circa 3025ish, probably didn't have a 1:1 relationship with
actual strength.
And a big part of why that's allowed comes down to the balance being disrupted. As long as the only threat to you was another Lord, and he had to do that same delicate dance with his vassals, you COULDN'T build your forces too large because the vassals might regard that as a (rightful) threat to their position and power. Once an outside threat upset that, however... the same vassals that might have banded together against you will BEG you to station more military forces on their land!
On the other hand, there IS a very handy and simple explanation for why the House Army numbers don't explode given the explosion in availability of new mechs: there are other destinations besides the House Armies for the new production and/or the old mechs the new production is displacing. The TRO Treadmill argument just on its own says that Occam's Razor says that there must be lots of mechs outside the hands of the five Great House Armies.
I recently read a neat, if bare-bones, article about why the B-52 is still in service despite three attempts to replace it - and a lot of it came down to "Well, the new kit was shiny and expensive, but it didn't
last."
Circa 3050-3060, it was an era of mass manufacture, with money and prestige to be made in replacing older stuff with newer stuff (or filling in on-paper strength with ACTUAL strength!) with plenty of actual expansion, but not a massive trickle-down - THAT would be going a little too far for one generation from neo-feudalism.
5) The mercs argument. Lots of sources covering mercenaries stress that there are many, many, many more mercs out there than the ones given writeups in that book. Most of those mercs are small fry/company sized, but they're out there. This alone is definitive, explicit proof that at least there are "a lot" of mechs outside canonical mention by way of the Merc Trade.
And a lot of those writeups also stress how short-lived most of those merc units are, with destruction or reabsorption by House units being their ultimate destiny.
I understand why you make the argument you do, because from a current point of view the tiny militaries don't make
sense, but if you look at it from the POV of Duke Sandoval it does. He has to make his vassals feel safe and protected while not letting them get strong enough to think they can hold out on him, and has to worry about the First Prince getting so much stronger than him that he might be replaced with a more... flexible... Duke Sandoval, that owes more to New Avalon than Sandoval.
'Mechs are power, and you don't go handing out power willy-nilly in a feudal system. Post Clans, most bets are off, but til then? The tiny militaries are perfectly logical.