So, the big rival character always has big bewbs, to make the female lead feel insecure about her ability to hold the male lead's interest. Normally, the female lead is relatively small chested, and this is easy to do, but what is a mangaka to do when the FL is massive?
Apparently, this guy's answer was "Eh. Add a KG or so of extra bewbie to the FL's chest size and roll with it."