The body in ballet is not arbitrary in its demands. The body in ballet and other dance is your instrument. That instrument needs to be well made and in tune. In ballet, it’s all about the line your body makes. Curves disrupt that line. Line is mostly about your bone and muscle structure which starts at your head shape and goes down to your feet even including the length of your toes. A Stradivarius violin makes a more beautiful tone than a poorly made warped violin no matter how good the violinist is.
FYI there are many professional ballet dancers with a B cup and even a few that are C cups. Professional ballet dancers are all lean long muscle with very little body fat. That in itself reduces breast size. There have been some dancers who had all else in place and were D cups or larger who have had breast reduction surgery. That can be said about the general populous as well but action to reduce that is not as prevalent in the general population unless size is causing back problems ,
Most in the audience who don’t have a trained eye my say they don’t care about the shape of the dancer but in fact that rarely is true. They are drawn to the long lean line and automatically assume that the dancer who looks like that is better, more graceful, more delicate, more ethereal, more pleasing to watch.