>> Is it a trend in anime to make the hot girls flat and the cold girls
>> round?
>
> I don't understand what this means.

In Plastic Little, Slayers and Tales of Phantasia, the girl who thinks most
about her breasts and sex (Tita, Lina and Arche, respectively) has flatter
breasts, while the girl who thinks less about sex (Ellyse, Naga and Mint,
respectively) have huge breasts.