I think Mr. Slim is much more filled out, he looks great! & Ms. Not So Slim looks fine too. Clowns live a long time, they may or may not grow at the same rate; gender? stunting? water quality? parasites? It's hard to say.
We do the best we can for our fish as we can & know better. You're doing fine! Relax & enjoy your fish, there is no "perfect"...Om...