tl;dr -> don't use bra sizes
Using bra size measurements is usually a mess.
The cup size is the difference between underbust circumference and bust circumference. (there are different measurement methods. One of them adds 4")
The bigger the torso, the less prominent they will look.
Every cup size is one inch OR two centimeters, depending on the system.
It usually goes AAA, AA, A, B, C, D... after that the letters differ depending on the system.
An American A is about a European AA.
An American B is between a European A and B.
An American C is slightly smaller or the same size as a European C.
An American D is about the same size as a European D.
An American DD is the same size or slightly bigger than a European E.
The band size can be stated in metric or imperial as well...
If the woman gets a bra with a half band size bigger, she might use a cup size smaller and vice versa.
So just from the numbers and letter, readers from different countries will get a wrong picture of the character.
Additionally, the bra size doesn't say anything about the shape. If the woman wears a bra, the tits could spill out or don't fill them. It could also be a pushup. If she is undressed, the tits could sag, hang to both sides and so on. The woman could also use her clothes to make them appear smaller or bigger (think of that Japanese 3D grid shirt)
And bra sizes are there to measure the size of
bras, not the size of
tits.
Basically, how do I accurately write about bra size when a character goes from having small breasts to enormous breasts? What should I be mindful of?
My recommendation: Don't use cup sizes at all. Rather describe them.
I think it is even better to use a pretty vague description than stating a cup size. Every reader will get his own image of it which might differ, though it will probably just what they want to imagine.
And most readers won't have an idea of how that cup size would look like. After all, you asked about it
