Ant is growing big already.
Insects don't grow obese (well they don't do in the same way we do)
It's growing at a fast pace
Is it feeding on Clayman's magicules ?
Most likely that it's a Queen, meaning Clayman is about to have thousands of little mouths to feed.
Edit: Actually looking back at the end of Chapter 15, since he was carrying the egg on him Clayman may have accidentally named the ant "Geld" as well, and that's why he passed out.