The reason is that the smaller the unit sizes for phalangitais the more penalties are associated with their formation. Larger unit sizes offer a large front, while still maintaining enough ranks to maintain enough mass to push against the enemy. Smaller sizes lead to exponentially more problems.

And because there are 20 unit slots in an army is not reason enough to max them out. 240 phalangitai costs comparatively more than 200 phalangitai would (we calculate cost by men), and to represent a successor army you would need less units than a roman army would need (raising a question of balance in recruitment, but we can't have it all now can we?). That you can have 20 units to an army, it does not follow that you should.

Foot