Forgive me for necroing this. If this has already been resolved or is no longer up for debate, feel free to delete this comment or whatever.

I believe we should use the rule34.xxx method of determining breast size discussed somewhat in this thread and this wikia section.

TL;DR They compare breast size to torso size instead of head size to compensate for disproportionate characters like Amy Rose. Their definitions are a little rough but we can hammer them out.