Good question! I guess it could be someone trying to say you look less like a cake, and playing with the adjective 'cakey'. I did find that this word is sometimes used in a sort of slang/dialect form to mean having a large bottom. So maybe it means that if you spend time cooking (as opposed to just eating take-aways, etc) you will be healthier and have a smaller bottom? And is also playing with this sort of dual meaning of cake/cakey?