I can see what you mean, one side seems bigger than the other. Could it just be the way that he is standing? A few more pictures from other angles would be useful. If he is eating and his behaviour is normal then I expect it's nothing to worry about. Keep an eye on him and let us know if anything changes.
I am relatively new to axolotls though, so it would be worth seeing what the more experienced keepers suggest.
Nice looking axolotl.
:happy: