... I was thinking more of some kind of genetic "OP potential" (using your word) that didn't necessariy have anything to do with the physical size of the body.

It could be that it's simply body size that matters (shrimpy Cyndane is still stronger than Graendal, so we're not talking about a huge reduction even though her body size changed dramatically), but when I developed the idea that's not what I was thinking. Like you, I'd prefer that body size was not part of the equation. But, I'd be OK if there was some sort of "OP potential" associated with each body, and Cyndane was either deliberately or perhaps simply unluckily put into one that had less potential than Lanfear's body.

I readily admit that this is a boring explanation, though. I'd prefer a more intersting one.