Ummm size 32 to 36 would mean she wants to add to her width of her body around. The cup size is in letters.
But, I see you are from United Kindom so I don't know if bra sizes are different there then US?
I am 18 years old and I am still a 36 A. Yeah, it sucks having small boobs, but I learn to deal with it. They have some pills and creams that are available to increase breast size, but I think they are a rip off. Also, if she works out her pectoral muscles, it makes the muscle larger, pushing her breasts out more. It also firms them up. I have also heard that after having a child, they increase a little. Good luck.
I'm thinking that its you mostly who wants to increase your girlfriends bust size, otherwise, why doesnt she post a question herself? There is only breast augmentation surgery or something called the BRAVA system which she would have to wear all day for months to see a result and is of a similar cost to surgery. Perhaps your girlfriend wishes your penis was bigger?