i think those people were always fat though
they were probaly very heavy even as infants and children. if my memory serves me right muscles theres a huge neruological transformation during early infancy where not just mental but physcial specializations are made so if you learn to walk early, are heavier than average when you learn to walk, or just have those specific genetics, you may have good calves.
im not even sure if this process occurs during the same time as walking though so dont take my word nesecerialy.
but either way, if youre a heavy kid then heavy teenager you have years and years and years of having your calves support weight all day long.
i definately training with heavy weight and probaly a lot of volume and frequency is the best way to develop calves.
i think the heavy weight may be the most important factor though, if you look at it anatomically youll see forearms are the upper body counterpart to calves and from my experience my forearms blew up just from holding heav weight so i figure since A) most fat people have big calfs (heavy weight under tension) B) forearms responding to heavy weight under tension the best way to train calves would be to use a lot of weight and a lot of tension even as far as to say to incorporate heavy static holds.