Why Does Breast Augmentation Also Increase Band Size?

That is an excellent question and one I wonder about myself. You would expect only the cup to increase with breast implants. There is no definitive answer and probably assocated with edema or tissue creep.

You are correct, only the cup size changes with an augmentation and not the band size if you are at the same weight. The differences may be attributed to a different bra type or manufacturer, or which clasp is used to fasten the bra strap

The problem with the cup size system is that it is not based on a pure volume measurement of the breast, so predicting the changes with augmentation can be tricky. In theory it should not change the band size. However, sometimes the bottom fold of the breast is lowered a little bit with augmentation so the band will be on a different level.