You can try to experiment with different foods but remember it may take up to a few months to see any results and switch them over gradually.

But I would think that you may have trouble no matter what type of food you go with.
It seems as if Sylvia's matabolism may be different than your other 2. You never know but logic says to me that the only way it would work is to limit her food or to keep them on different diets.
But since you did say that CSFTDLS makes some dogs fatter it may just be that food, but why wouldn't your other 2 be fatter too?

California natural is a good food too.

I feed mine Nutro natural choice & never had any problems.