I am not a big person, but was wearing a size 10A (32A) bra for years because that's what I wore through high school. I thought that the underwire cutting into my right breast was because of my scoliosis and normal for me. I went up to a 10B (32B) and started massage and improving my diet (also have been noogling for 2 months and I am finding that I am now in between a 10B and 10C (32B/C). So there is a lot to say for getting the right sized bra!
The only thing I find confusing about fitting is with sister sizes, when you go up or down a band size, you also go down or up a cup size respectively. Yet with the "inches rule" described in the links for fitting, when you go up or down a band size, you should go down or up two cup sizes since the band sizes differ by 2 inches. In my case, my underbust is 30 inches and my overbust is 34.5 inches (lets just say 34 for argument's sake): This would put me at a size 30D, or in a 32 band, size 32B. Yet the cup volume is larger in the size 30 band. I know it's approximate the the sister sizing and inches rule contradict each other, which makes it annoying to buy bras online I guess. I only bought one and it kind of fit (the band was too loose).
The only thing I find confusing about fitting is with sister sizes, when you go up or down a band size, you also go down or up a cup size respectively. Yet with the "inches rule" described in the links for fitting, when you go up or down a band size, you should go down or up two cup sizes since the band sizes differ by 2 inches. In my case, my underbust is 30 inches and my overbust is 34.5 inches (lets just say 34 for argument's sake): This would put me at a size 30D, or in a 32 band, size 32B. Yet the cup volume is larger in the size 30 band. I know it's approximate the the sister sizing and inches rule contradict each other, which makes it annoying to buy bras online I guess. I only bought one and it kind of fit (the band was too loose).