I believe you are running under a number of misconceptions. Two large ones are:
1) Where did you get the 3.2 million from? 31st century numbers can't really be used it as a measurement for a Reunification War era discussion given the fallout from the Succession Wars (raiding, warfare, loss of technology, loss of trade, etc). As far as Lushann is concerned, it has an arctic climate with most of its population living underground. That doesn't exactly sound like a planet people gravitate to. Just look at Hesperus II. One of the most valuable planets in the Inner Sphere and yet there are plenty of planets in the Periphery whose population dwarves Hesperus II's.
The 3.2 million number has been more or less consistant from the first periphery field manual to the most recent books. Also, the reunification war OA was specifically fluffed as being smaller, with much of the immigration occuring later, and in addition, as was said, the OA managed to get a negotiated treaty, avoiding some of the devastation that hit say, the TC. For Lushann, you're making my point-- Hesperus II is a factory--one that is more or less supported by a large, intersteller polity. We can say that yeah, there's production at Hesperus, because you have the Lyran's shipping material to it, because again, it's a factory. But the OA doesn't have that capability-- it has the organization of a particularly passive amoeba. So every world, more or less has to survive on its own, and 33K would barely be able to support itself, let alone an industrial facility AND a petrochemical extraction plant.
2) Why wouldn't the OA grow "during the period after the reunification war" when it was under the control of the Star League who would bring terraforming, agricultural and medical technology into the OA? They were the only Periphery realm to be able to negotiate a peace treaty with the SL rather than have terms forced upon them. Forlough was transferred to the Taurian theater before the end of the war and never returned to the OA so I'm not sure what effect you are thinking he could have had.
If the OA grew after the period of the reunification war, then during the war it didn't need Forlough to conquer it; it needed a boyscout troop. It wouldn't have enough economic power to support a platoon of technicals, let alone regiments of infantry and armor.
3) The biggest factor that limited the OA's growth was the fact that the OA's federal government had very little power and very little funding to get anything done. It is comparable to the early years of the United States where the states were extremely reluctant to give the federal government any power whatsoever.
The capital world of the OA had a smaller population than the united states--circa 1800. And yet, this is with A. An agrarian life style that we know tends to produce dramatic populuation growth, on a world that has avoided general war for hte last three hundred years (sure, we can assume pirates cruise by and lob salted nukes at the world, but that's never stated). B. on worlds with little technology and as you said, a desire for self-sufficiency, which means that by definition, the worlds are more or less habitable; ones that were not would have gone the way of all such worlds, or more likely would never have been settled in the first place.
To show how bad it is, the population of the capital and most of the other listed worlds would be under the *prehistoric* population of Terra.
The problem gets worse when you look at Praxton, Ferris and Quatre Belle--worlds with higher populations, ranging from 14ish million for Quatre Bell to over 150 million for Praxton. You wouldn't have laser manufacturers at Lushan, because A. Lushan's population couldn't support an auto-shop, let alone a major industrial manufacturer, B. the OA doesn't have the political organization or logistics capability to handle a Hesperus style industrial world, C. if the OA doesn't, as has been said, have decent political organization, then only the higher population worlds are likely to have the banking and support industries that make weapons manufacturing possible.
Economically,
Even given fasanomics the OA makes little sense.
Ntoe that upping the population doesn't change the character of the OA. If you say, change Lushann to 10 million people, and Alpheratz to 30 million people... well, you still have worlds that are emptier than earth was in 4,000 BC (by some estimates. Less optomistic estimtes would have the population equivelant to earth in 2500BC). Or to put it differently, you'd have a
planetary population that would for Alpharetz be equivelent to the US population in 1860. Tiny-- tiny enough that finding land would still mean "drive three days away from wherever you live." But at least large enough to support the industries that were stated to exist in the OA.
TL: DR, I've taught
Fourth Graders who know more about demographics than FASA did.
Edit, now looking at the publication history, there may be some "soft" retconning going on. The worlds with higher populations-- Quatre Belle, Wynn's Roost, etc, are mostly worlds that have been published after the origional FASA run, while the current low population worlds are all worlds that were either published in 1E Periphery handbook or at roughly the same time. I forget that the policy was on actually changing previously published hard numbers.