You ARE NOT a bra size, I think this is the first step towards understanding bra fitting.

This means, that you are not universally a 30G, you may happen be wearing a 30G to fit your breasts, they are a determined size and you can always check that size by looking down and taking a look at them .

You are not a bra size, means that you are not "a 30G", you will surely find a 32FF or 28GG in a different style/brand to be more comfortable for some situations (or time of the month). Breast size varies more frequently than you'd expect. So remember, you are not a bra size.

You are not a bra size, means that the bra size does not determine who you are, it's just an ( apparently random ) identifier for further reference and a similar bra on your next purchase.