@Danholm,
Simply put, this is about control. Want to or not...aware of it or not...you're having a power struggle. Shaving your head is not radical. Changing your body permanently with breast augmentation (though beneficial for some through improved self-image), is permanent...and is a serious personal choice. the fact she is not funding it herself, it it requires you to fund it, SHOULD you DECIDE to vote no, you should be able to do so without repercussion. If she wants to have it done, she should fund it herself. If it is just cosmetic...(because of someone's insensitively with a joke), then she shouldn't be asking you to fund it. If it was due to a deformity or a serious issue, there'd be no doubt about what to do.
However, if one breast was decidedly different than the other, that would be a different issue. she has stated her reasons and they appear to be NOT about the non-uniformity.
This is a petty response when she says she'll break up with you if you shave your head. Not too mature..or perhaps that committed if that is her response.