Buying bras that fit correctly can be incredibly frustrating.
Don’t give up because it is essential to have a good fitting bra in order for the girls to not flop around and get all saggy. If the sizing were simple, straight forward and logical, the sections of stores which sell bras, would only have to sell about dozen different ones. As it is, there is such a VAST variety of sizing criteria, that better clothing stores employ special clerks to help each woman to find the right “fit” for her specific unique body.
There is no specific criteria which can define the size of a “C” cup on *any* bra. There is not much of a criteria for band size numbers, either, for that matter. Going from a 38 to 40, same manufacturer, same “line” (same model/style name of bra) only generally means the chest “diameter” has increased two inches — that the measurement of the bra band is [kind of, sort of, maybe,] two inches longer. From brand to brand, from style to style within the same brand, the band measurement can vary even when a bra has the same band numbers.
Having breast growth, should not really change the band size; it is breasts which are growing, not chest size. While you are growing, the best bra is a training bra, which for an adult is a sport’s bra. “Fit” means the combination of a band number, and a cup-size designator.
Cup sizes are a proportion of the band size. Cups on a 40B bra, will be significantly larger than the cups on a 32B bra; same manufacturer, same model/style name. Going from 38C to 40C the breasts have to be bigger even though the cup size designator remains the same: the cup will be physically larger.
Going the other direction with finding a good “fit”: If you go up in band size (e.g. from 38 to 40) the required cup size for you should go down (e.g. from C to B).
As a generality, the more expensive a bra is, the more accurate its band measurement will be…
Ignore the web sites which try to engineer a size chart. Even Penney’s, etc. web site, cannot generate the best bra size for each person. Those charts are for basic guidance only. They establish a place to start.
Which also means that it is VERY important to ignore the TG/TS web sites where people compete with each other about breast growth, or do male-style bragging about breast growth, based on bra sizes. It is all essentially meaningless.
If you find a good cup size, but the band is too small to fit around your chest, it is possible to buy band extenders; they are are like using an extension cord to further the reach to the bra band.