Anyone who is a bodybuilder training for strength will always have a layer of fat because you have to eat a ton to gain and support that much muscle. Only pansy show off body builders are ripped because they specifically eat less to lessen body fat.
Pansy show-off body builders? Haha, no.
But as for baseball players, the extra weight is particularly beneficial to power hitters. They often play positions that don't require much running (i.e., first base), so they're able to pack on a few extra pounds to help their team offensively.
Even the ones who do move around a lot (e.g., outfielders) generally do so in sprints, so the upsides of having the extra weight generally outweigh (hah) the downsides.