I believe they were removed due to the Behemoth being huge and the little laser dudes were already there as flying enemies.
And by Behemoth being too huge, it might have lagged the game really bad if there had been two at once.
Size matters not. If they don't have many polygons on them it won't matter the size. Size of the polygons are just coordinates. What would make a difference is how many polygons were on there which weren't a lot.
Plus even if it would cause lag they could've easily make 100 soldier Heartless and have them respawn while the background would have sprites of a battle to create the illusion you're fighting an army. They did something similar to that in BBS when you had to fight an army of those floating mushroom unversed.