Rj has placed some emphasis on the body as the vessel for a channeler. SH also mentioned this to Aran'gar when he complained about being a woman.

It is possible that a channeling body can host most channeling souls, but as you get to the highest strengths you need to start being careful. Aran'gar has a strength close to the top, and so does Osan'gar - so perhaps a good strong body is enough. However, perhaps a perfect body is needed for the highest strength possible, or a slight reduction in strength occurs and you end up with someone like Cyndane who is marginally weaker.

Look at the trends. None of the thirteen Forsaken are short or in feeble bodies, and that applies to all the high strength channelers on the Light's side too. Then look at the top strength level : Rand is very tall for a man, and very strong, and so was Lews Therin. Ishamael was also tall. One thing that's impressed upon us about Moridin is that he is very tall, though his chin is a little strong. Lanfear was tall for a woman.

It's possible that the highest levels of strength need a body that is 6" in height. Cyndane is only about 5" in height, so that might incur a 5% strength penalty. It's old news that the body fatigues as the Power flows through it, so it wouldn't be a leap at all to assume that smaller bodies just cannot handle that level of saidar or saidin. A body like Moiraine's might be incompatible with a soul like Lanfear's, but it can hold Moiraine's strength.

I would be surprised if this is the case, only because it's a little convoluted. However, I do think that there's a strong case to be made for this theory because it is probably the most likely. Cyndane is the only short and petit Forsaken around, and suddenly she's weaker. It's a logical train of thought.