Bust girth is NOT a reliable indication of bra cup size.
The pictures here depicts an experiment that will lay to rest the misconception that bust girth can be used to predict cup size.
Measure the bust girth of someone in any bra. Notice the distance between the apex of one breast to the apex of the other. Pin or pull and hold the centre of the bra so that the distance between the apexes becomes smaller and remeasure the bust girth. The first bra illustrates the bust girth that this bra creates left as is. The second picture illustrates how decreasing the distance between the breast apexes makes the bust girth measure smaller. The cup size has not changed. The bra has not fundamentally changed. Only the apex spread has changed. Try the experiment yourself and let me know if you disagree.
Those designing minimizer bras take note: A simple change in apex spread can be used to create a minimizer bra. This does not necessarily need to be done by a change to bridge width, and usually I advise against. Offsetting the apex on the cup pattern is an effective way to shift breast mound volume, decrease apex spread and create a smaller bust girth.
This link provides more information on why bust girth is NOT an indication of bra cup size.