This is a really bad handwavium.
Of course it is.
That does not however make the generalisation of the analysis less true.
From this we see that RMN is already NOT building as large of an SD hull as they COULD if they were willing to limit the acceleration of their hulls. So, lets not go too deep into hubris here. Manticore is already making the REAL choice of #hulls compared to maximum destruction power.
That´s not really true. They haven´t really had the time to figure out where the "next step" should be, how much of a reduction in acceleration is acceptable to get more combat power per tonnage, so far they have only come far enough on that road to get the new extra large cruisers and battlecruisers.
Give them a year or two without someone breathing down their necks and there will very likely appear a new SD that is a big chunk larger than the current ones.
Designing a completely new weightclass that has NEVER been done before, it´s not a quick and easy thing to do.
Look at real world navies in the early to mid 20th century in comparison, a time during which tonnages jumped a lot, there are some really, REALLY, bad designs to be found there, because they were not really sure how to use the larger size properly.
The correct analysis would be, what sized hull can carry the necessary offensive equipment, active defenses, passive defenses(armor schemes for capital MK-23 missiles), and ECM?
Are you serious? No, no, NO!!!
That´s what you do if you want useless, overcrewed and expensive abominations.
The above completely ignores crew requirements, the ability to load up on armour(the BIG reason why SDs survive better than smaller ships), salvo sizes and more...
And for wallers, numbers are effectively irrelevant as long as you can deploy them in at least squadron size.
Building smaller than you can when you´re already building many dozens would just be foolish.
Building small when you have a lack of crew and especially officers? Madness and folly.
Offensive missile loadout is approximately 25% of the hull volume.
It varies. That´s one of the things with building bigger, you will be able to increase the weaponload % if you wish. Because, among other things, total crew per tonnage is smaller, sensors and ECM per tonnage is also smaller. Needed stuff like boatbays and pinnaces loadout takes up less of the total.
And so on. The smaller you make the ship, the greater % of it will be taken up by all those things that does NOT scale down with a size reduction.
For light units, that is a penalty you have to accept because light units main requirement is numbers, that it exist and is operational.
For wallers, maximum combat power is the only thing that matters once you´re building at least squadron size.
Total pod #'s is the single largest determining factor in ship total tonnage.
Maybe. But it may not remain that.
Due to nature of Apollo FTL, no true viable defense
Sure there is. If for example you added 2Mt of armour and counter measures, that would drastically increase survivability.
Maybe not as maybe one just needs twice as strong sidewalls as this is the MAIN armor.
It´s the main armour for NON-wallers yes, but armour has always been stated as an important part of why wallers can survive hits that would shred other ships.
Anyways, lots of unknowns and never will be known, so throwing a blowhard number of 25% more effective with no framework is laughable.
Not really no. It´s a guesstimate based on known facts. And it´s not meant to be a hard or precise number.
It was meant as a way to compare, if you build smaller enough ships to get an extra 10% greater numbers of wallers, how much would the difference in combat power be.
Sorry, can't stand safehold. 
Guess will just have to put up with the juvenile John Ringo Zombi Apocalypse stuff. Quite humorous actually.