[JRDeBo]

I voted for RPG myself (also a few others).

Whenever I thought of this I always saw the BE as being both a hindrance and an aid at the same time. Larger breasts would drop evasion/agility/speed making it easier to get hit and criticals more likely, but in exchange strength and int go up. As you level up the penalty decreases allowing you to go larger, or you could choose to keep them their normal size and play a defensive build. (balancing this part would be the hardest, because it would either make the early game too hard and the end too easy, or make the whole system pointless if it doesn't affect enough.)

Enemies would obviously be able to use skills that alter your size. You would be able to adjust your size in battle too (at the cost of a turn), the amount you can adjust per turn will be small at first but increase as you level up.

Equipment would work like standard RPG equipment boosting stats, though I'm sure a few special items could be added to either increase your bust without penalty, or lower the penalty cost for growth. Cursed equipment might cause random irreversible growth until you get it taken care of.

Of course one thing that would need to be figured out is if this would be an RPG with BE elements, or a full blown hentai RPG, the first would have more traditional enemies/skills/items, the second would have more "unique" ones and give more leeway to get away with stuff.