At the moment it's just random. I have vague possible plans to have a way to influence it, but I don't have any guarantees at the moment ^^;
Perhaps eating different foods could affect Ebony in different ways to set that up? Example: Donuts trigger butt, milkshakes trigger breasts, and increasingly fattening meals trigger increasingly rounder sphere modes.
Something like that could work, I'll think about it! ^^
Alternatively, you could have the degrees of spherical progress as she grows.
