If Aran'gar is much smaller than Balthamel. And as said, Balthamel's body might already have been better than what he actually needed to access all of his OP potential.

But if you would put super strong channelers like Lanfear, Rand or Ishy in a much smaller/less capable body, it might reduce their strength somewhat.

Although not perfect, this is IMO the best explanation we have yet, because the others make even less sense or were ruled out by BS.