to the theory that there's nothing physical about strength in the power, or skills.

Personally, I think Jordan sets this up so that all genetics does is give or not the ability.

I believe the rest is all about the soul.

My personal theory is that a soul's ability to manipulate the "divine essence" of Creation (the energy form of the Creator himself), and at which strength, most likely depends on a series of factors, one of which might be something like "strength of will", another how old the soul is (how many lives it has lived) - or how much it has "progressed" on some unknown scale. The progress in question could be "spiritual" or perhaps even related to a soul's thirst for power (that would explain the top Forsaken, especially Lanfear's whose power thirst is unsatiable and who has reached very high, perhaps the limit in strength), and whose deepest wish seems to be to reach a divine status, make herself an entity capable of defeating the DO and the Creator themselves.

The factors may not be all present in one individual, and there might be many different "paths" to growth of a soul's strength through the Ages. Perhaps you can regress as well.

Based on the strong personalities (and usually, strength) of all known sparkers, I suspect this is also the strength of the soul, or its will to channel, which determines if one becomes a sparker or a learner.

The strength of the souls of channellers might explain why Min sees so many Viewings around them (and saw something similar for Birgitte)

Rand's soul, for instance, is probably as old as the Wheel or so. He is the strongest male channeller known and has a very strong will and moral fiber, and he's possibly as strong as any man can get (and thought equal to hsi previous incarnation as LTT). For all his weaknesses (some of which are virtue, like his deep desire to protect the lives of others, even though he tended to turn it into a flaw by misjudging women to be weaker), Rand showed a formidable strength of will to resist the Shadow. Most people would have given in to Ishamael by now.

His antithesis Ishamael is nearly or equally as strong, and he too is possibly very old. Lanfear's archetype is also one of the most pervasive female archetype (or aggregate of related archetypes) in all mythology (from all cultures and eras). It's not really a stretch to suspect hers might also be a very old soul - and she happens to be the strongest female.

A lot of the other strong channellers also have a very archetypal nature, and are present in some form or another in many legends (that's true for Graendal, Alivia, Nynaeve, Egwene, Elayne, Talaan and so on) and pantheons.

Of course, there are non-channellers in the series who are almost as archetypal, and my feeling is that theirs may also be very strong souls, but currently incarnated in non-channelling bodies. Mat, Perrin could be like this, and Birgitte.

Down the line, you find souls that are barely beginning to touch "the divine", like Morgase's, or Daigian's. Possibly, there are a lot of souls that have not even reached the threshold where they can touch and use the Power.

I don't think that it's a coincidence that after a long hiatus in the third age, some of the very powerful souls, quite a few with special talents, have been reincarnated, like Nynaeve, Aliva, Elayne, Egwene etc. in time to take part in the last battle. I think the Wheel or the Creator himself kept more and more of those souls "in store" in the soul pool, to have them available for the era of the LB.