Ball Python getting thinner between feedings?

My ball python, Abbott, eats hopper mice. He's not much bigger than they are, but he gets noticbly thinner a few days before feeding. Is this just him pooping, or should I size up his food?

When their food finishes digesting, they do appear to be thinner than right after eating, if that's what you mean.

How much does he weigh? Mice hoppers are only fed to hatchlings, and they quickly bump up to adult mice.

Weight??