The making of gateways, like shields, is directly related to strength in the power. There's abundant proof of that. But like every weave, it's also a matter of skills.

And just like shields, an individual can break all the rules if he or she has exceptional skills (a talent) with those weaves. The worst you are at making a weave, the more power you need. Androl can make gateways with far less saidin than most people, that's all.

It's almost a constant with the OP: for many weaves, lack of dexterity or skills/talent can be compensated by using more Power, if you're at least able to make the weave (if you don't have enough ability to handle a required flow etc, you're screwed even with an angreal). It's the same concept behind the equality of men and women. The average woman needs to use less saidar than the average man needs to use saidin to achieve the same result, because on average a woman's weaves are more "energy efficient". This increased efficiency comes from their average greater dexterity at weaving.

So the size of gateways is directly related to your raw strength. It's just also related to your skills with the weave or the abilties behind the weave. It's not one or the other, it's both. It just seems like the skills of channellers with Gateways don't vary massively. The majority of the channellers are average with the weave, and the difference between their gateway sizes rest mostly on their strength.

SIuan's dexterity has not changed, and her current strength is enough to make her Healing weave work. It just means she needed nowhere near her full strength for the basic Heal-All weave before her stilling, that's all.