User blog comment:Kaido King of the Beasts/Chapter 811: Homeland/@comment-111.119.36.194-20151226161006/@comment-24769684-20151226214253

I expect Big Mom's dependence on candy to come into play. Perhaps she ate Caesar's candy, much like the kids, which has made her enormous. The reason she demands more and destroys islands for it is because she's simply addicted.

Of course, that would mean everyone who feasted on Fishman Island would grow big, but perhaps normal candy works as a substitute?