The differences between the news bodies for Moridin and the 'gars weren't that big, compared to the difference between Lanfear's and Cyndane's body, especially considering top Lanfear's OP strength (besides, Moridin even got a stronger body than Ishy had).

The idea is that if you have an exceptional potential you need at least a certain capable body. For example, Ishy's body was enough to allow him to use his whole potential, but if Ishy had been reborn in the body of a very small man, he also would have lost a part of his strength. On the other hand, Balthamel didn't have such a big potential anyway, so he can have a smaller body without losing strength.