But if you put an axolotl with small gills into a warm, low oxygen environment, they won't necessarily grow bigger gills. Put it this way: the gills grow as big as they're going to grow as an axolotl ages. They don't get any bigger. However, if you put them in high oxygen, cold water, they can get SMALLER. It just doesn't work in reverse. They may get a little fluffier, but they won't grow huge.
The reason Stubs is small is that he has to spend all his energy regrowing his limbs and gills, instead of growing in length. He'll eventually catch up.