@Mitsu the "muchi" in "gachimuchi" stands for "chubby". The softness of male pecs (pectoral muscles) or breasts comes from the amount of fat in the area. The amount of fat really depends on the "discipline" of a "muscular man". Bodybuilders tend to optimize the "looks" of muscles, showing them off, so they tend to minimize the fat, leading to more rigid breasts with softness only coming from relaxed muscle tissue. Fat is a requirement for healthy and strong musculature, so "strong man" archetype/disciple will be very soft (but also chubby and not muscular-looking). Everything else sorta falls in between. Oh and it also depends how relaxed the muscles are, naturally.
Maybe local medicine/physiology experts
@lebronjames75 and
@moth_ball can add up or correct me if I'm wrong.