I can see a body having "less genetic potential", which wouldn't depend on how big of a body the soul was placed in. Assuming the size of body is limiting seems random. Far more likely IMO that the body either has the correct genetic markers and can channel to the full extent of the souls ability or the body cannot channel at all, but I can see there being shades of genetics playing into strength too. I just seriously doubt it has to do with how tall someone is!

Halima would be dramatically smaller than Balthamel, on a % basis far more of a reduction in physical size than Lanfear to Cyndane so regardless of how much less potential Balthamel had (and by your thought he could have been a 100% potential soul and his original body could have been too small to contain it all too) there is no way Halima's body could have contained all his strength. He was certainly stronger than any woman is likely to be in raw strength.