Where in any of the games did it say that Japan had been invaded before the draft age was changed (not saying it doesn't, it's just I don't recall it at all)? And besides, the Honor Guard isn't part of the main army, so it may have just been the case where only the regular army started after they'd been invaded. Also most of Asia IS gone, since if I recall correctly, they pointed out in episode one that they had recently lost the Korean peninsula. As for Europe, I don't know since the Eurofront is pretty much the one Muvluv chronicles/spinoff I haven't read at all.
Putting aside the random resizing due to anime just always being inconsistent with perfect proportions (Gundam does it all the time :P), Destroyers are just about the same size as a TSF. The Zuikaku is slightly shorter than the F15 depicted here, so it should be about the same height at least.
Also I'm assuming you've never played any of the games in regards to the instructor's Type 94 Shiranui having 3 guns despite one of its arms being blown off. Those back-mounted racks can pop down and be used to fire the weapons mounted on them on the Shiranui and a few other models (I don't know with 100% certainty, but I don't believe the design of the Zuikaku/F-4J leaves enough clearance for the guns to pop down all the way).
Example image taken from the game itself: