Why does this alias exist? There's a difference between breasts that are small and no breasts. I would consider post #609712 and post #609489 to be in the flat_chest category, but post #610114 and post #609342 are hardly flat. The two rules I would provide for determining between small and flat are 1) if you can cup your hand around it it's not flat, and 2) if you look at her from the side, if she's flat only her nipples should be sticking out.
Updated by jxh2154