In the sports where excess fat is an advantage (hammer, shot, to some extent discus) here is why:
shot: putting the shot mainly involves acceleration in the vertical direction, i.e. pushing the shot upward. High cross-sectional mass is a big advantage, favoring thick-trunked endomorphs who tend to be fat.
hammer, discus: considering the rotating thrower and implement as single spinning object, the concentration of mass in the center (thrower's fat gut) lowers the moment of inertia relative to the system's overall mass, so that it is accelerated more easily.
Javelin doesn't favor fat people. And baseball pitching and cricket bowling especially doesn't. Shot, hammer and discus are weird.