Body difference

It's called Twinkification or Bottomification. It's a sad trope in manhwas with Muscle bottoms. They are with huge muscles but as they story progresses they turn into stereotypical bottoms.

Here, it makes a bit more sense since when the story started Ho had just resigned as an active-duty soldier, so he had the muscles to match. But now he is just some writer/single dad living in the countryside, so he doesn't get the diet/exercise etc needed to maintain that army physique. He doesn't work on farm, outdoors like Haebom either, so it makes sense that he is losing his muscles and getting a leaner body.