The Inner Sphere is, supposedly, 'militarized'.
But when you look at the actual numbers compared to current day Earth, the factional armed forces are very thin. The caveat is, of course, that we mostly talk about the famous and the infamous.
The U.S. Army has approximately 12 Divisions. 12 Divisions is roughly 36 Brigades, or maybe call it 50-75 odd regimental size units, if we suss it out by the standard 1 to 3-5 ratios. Granted, it's Earth's most expensive force, counting in the other branches, of course, but the whole U.S armed forces is not the largest by manpower. China has 2+ million under arms, now add in India at least a million, Russia, North Korea, Pakistan, South Korea, Turkey, etc.
It really starts to add up.
But then we go to the BTU. Granted, the populations of most worlds are much smaller, but there are factions with a number of planets that can reach upward of 100 worlds to counter smaller populations. I'm not doing hard calculus here.
A major capital world or important strategic place would probably be considered well-defended if it could boast what? Lumping BattleMechs, armor and infantry together we typically count 5 to 8 regiments plus maybe 3 regiments of militia for a grand total of 10 to 12. So, if I'm invading New Avalon, and the Davions are ready, and I'm looking at 10-12 regiments of defenders, we're talking a major event in the timeline. Most of the time, it's maybe half those numbers for a notable battle and the very rare Tukkyidd event where it's a massive pile-on of everything a faction's got.
So, my point is, when we talk about the BTU, I think we're talking about an OPTIMISTIC universe where the population to military ratio is generally much, much lower than it is on Earth at the current time. Yet we tend to talk and write about it as if it were heavily militarized. What it does have is eras with the heavy use of weapons of mass destruction, which on Earth would be a game-ender, since it's only one planet.
