>>280594618
One possible reason is that mangaka and animators have tight schedules, and anatomically detailed muscles take a long time to draw. Another is that many protagonists tend to be quite young, while men usually peak in muscle mass between their late 20s and mid-30s.
But besides that, I would like to question the initial assumption. If we look at the best-selling ongoing manga and their male protagonists, we see they usually have lean, athletic builds.