Since you already have your girlfriend's vagina, there's no reason not to try her breasts.
You go around the panties on the floor to pull a pair out of your girlfriend's bra. Even though your girlfriend has B-cup breasts, it is difficult to put the bra on for your frame.
It makes you wonder, if the act of "wearing" can't be achieved, can the magic still work?
You put the bra on your stomach, which is the only place you can barely fit into your girlfriend's bra. But instead of rushing to button it up, you try to press the bra hard against your lower abdomen, simulating being too big to wear it properly. The magic thing happens, though you're not actually wearing a bra, but your lower abdomen gradually bulks up a set of fat, the excited touch reaches your mind from your lower abdomen, you remove the bra and see a pair of developing A-cup breasts clearly above your abdomen.
You walk around with your head down, watching your breast shake slightly with your movements, which gives you a slight thrill, but it makes you laugh more because it looks more like cow's udders than human's breasts.
And you realize that, even without the act of "wearing", magic can still work to prevent the inability to wear due to body shape.