Quote Originally Posted by shazam View Post
Well maybe not exactly the same size as the rest, but one moment he'll be massive so that the other autobots can sit inside him and then the next minute he's only a bit bigger then the other autobots.

I guess, its kinda like the situation with Megatron when he transforms into a gun.
'Mass displacement' explains it all.
There's no Wiki entry on it atm but someone should sure make one.