Aeon's get a boost in stats every (30 I think) battles until 600, when they stop getting the bonus. The buff isn't huge, but it does add up.
Other than that, their stat growth is affected purely by Yuna's. It isn't a 1:1 ratio though - as an example, you may notice that raising Yuna's Strength by 4 causes Ixion to get 2 Strength, 1 Agility, 1 accuracy, and 50HP. Aeon's all have a predetermined stat growth anyway - while boosting Yuna down Auron's path will cause their Strength to increase quickly, it'll eventually level out, so then the only aeon with 255 in all stats when Yuna has 255 in all stats is Shiva (though her HP will be lower than several of the others).
In the book, it says He created us to be just like Him. So if we're dumb, then God is dumb, and maybe a little bit ugly on the side. - Zappa