What bra size could also fit?
The so called sister size , sometimes also called diagonal size, of a bra is not the same size. The sister size is the next smallest or next larger bra size. It only varies in terms of a wider or tighter under bust circumference.
So if you feel that the under bust band is too tightyou can try to take the next size up of the under bust sizes with a smaller cup size: if your size is 36D, then size 38C has the same cup volume and a looser under bust.
If you think that the under bust band feels to loose, take the next smaller under bust size with a bigger cup: if your size is 40B, then the size 38C has the same cup and a tighter under bust.