Keep in mind this is NOT a "do this, not that" situation when drawing, more so a reminder of what to keep in mind when making character design choices. If your character works out/fights a lot but isn't a gym rat type/isn't super invested in how his body looks, they're not going to look like the body on the right. Again, I'm not saying its wrong to draw the right body, it's ok to do it, especially if you're going for a superhero/superhuman look. But if he's just a regular guy who happens to work out a lot/is athletic, the left body is more realistic. Hope this makes sense!