One thing to consider is that even seven-times-Rejoined (Ardbert wasn't part of us at the time), one Ancient that mistakes us for a child can't even detect our ability to use Creation magic, it's so small. You'd think that seven-times-rejoined would mean that we'd have half the capability of a fully-rejoined Ancient - but that's clearly not the case. Additionally, beings on the First, each with a seventh our "soul density", can put up as much of a fight as anyone on the Source.

My guess is that there's some kind of tipping point, or some kind of asymptotic curve; power levels are relatively flat until you reach a certain point, beyond which you quickly become godlike. Conveniently, we will never see that tipping point come into play.

This theory does make one wonder, though, why Hydaelyn split the world into fourteen, if folks would effectively be crippled just split into two. Perhaps she had no control over the number of pieces into which she split the world, or perhaps Zodiark's tipping point comes much sooner, or maybe she anticipated that some misguided souls would try to glue the world back together again and wanted to make it harder for them to do.