Gills are a difficult indicator.
Small gills could mean that the axolotl isn't feeling well, that his skin is irritated, or that the water quality is so bad he gets most of his oxygen by gulping air - all of which are bad.
Small gills could however also mean that the axolotl is very relaxed (so his blood flow is low and he doesn't need much oxygen) and that the water quality is so good he can get enough oxygen without big gills - all of which are good.
Similarly, big bright gills could mean an excited, healthy axolotl who doesn't need to gulp much air; but could also mean a stressed axolotl, agitated skin and water without much oxygen.
And then there's also a genetic factor - some axolotls just have bigger gills than others.
So the gills alone aren't much of an indicator. That also mean the gills alone aren't anything to worry about, too.
As long as the water parameters are good, the axolotl seems relaxed and healthy, and there are no sudden changes in the gills, there's no reason to worry. If they've been this way for as long as you have him, that seems fine to me.