I got my gallbladder taken out and my stomach was lopsided afterwards, so it makes sense that once the baby's out his stomach would get flatter.
It's just that after the baby there's some fluid, the placenta and there's also some bloating and inflammation so in real life it takes a couple of days for the belly to loose some of the volume
But then again it's a comic with a demonic magical birth so
Im not an expert but there is something called after birth thats basically after the baby is removed the belly of the mother still have to push out a lot of things and go back to its normal shape, it doesng happen immediately
But this is a fictional story so my comment was mostly just me joking not serious
Also he's a dude. It was probably a magical uterus. What women have is a death trap.
He even removed the belly bump in just one frame, incredible