As far as I know the humps are used for nudging females during the breeding process, as well as to attract a mate.
I know a lot of SA/CA
cichlids get them. And just a handful of African cichlids do.
I am pretty sure the size is determined by dominance/genetics. For ex. My RD has a slight hump, but... When I add feeders or target fish it will grow twice or more in size. Being that he is a single fish with nothing else in his tank his hump is usually not at a noticeable bulge.
It is also not uncommon for dominate female cichlids to grow a hump as well. I used to keep dovii a few years ago and my female had one, (not impressive by any means but she had one).