BE Witches has the problem where bust upgrades are permanent, and the gaps between each upgrade is pretty far, so I've always considered it more of a "big boob game" than a BE one.
If it was up to me, upgrading your bust levels would just increase the max size, and your bust would adjust based on, say, mana or something.
Damn great game as a "game" and I'd highly recommend it for the overall feel to the art, which kinda sucks since it's carving out an annoyingly limited niche:
1. It's too lewd for SFW fans, and the name alone means they'll have to play it in Appear Offline mode.
2. BE fans get a bit disappointed by how little actual breast expansion scenes are in the game. (It's basically flipping a switch off-screen and only the MCs get an expansion animation of any kind.)
3. Big boob fans get too impatient waiting for the first bust size upgrade, which takes a while, and all the BIG upgrades are WAY later anyway, the first upgrade isn't really noteworthy.
EDIT: don't get me wrong, it's a great game, it's just that it's carving out a difficult niche.