The basic idea is that a specific soul needs at least a specific capable body to access all of its OP potential. It's e.g. possible that a body like Aran'gar's would be the minimum for Balthamel to access all of his potential. Or Aran'gar's body might even be better than what Balthamel would at least need.

Contrary to that, Lanfear needs in order to access all of her top strength (apparently the strongest a woman can be) a body, which is somewhat bigger than Cyndane's. Of course, that such a power drop happens anyway because of a transmigration must be a very seldom exception, otherwise all Forsaken would know about it.

However, this mystery is certainly something that will be revealed soon!