Well, Grundy is decent sized in his own right(that's what she said), and if you haven't seen the pics of Bane and Lex...Bane isn't that big. Not like Arkham City where he's double the size of Batman. He's closer to the size of Lex's power suit. So I doubt that's a problem. Bane really isn't THAT big in most cases. I mean, he's been portrayed as such in certain circumstances, but not always. Besides, this is a different game. Bane would fit in with the epic over the top super hero battle they are going for.
Lastly, Goro and Kahn have been playable a few times(Trilogy on PS, MK

on Gamecube, and MK:A come to mind). So it's not impossible. They mainly just don't like their boss/sub bosses of that game playable.