Can girls boobs get smaller?

There are a couple of girls at my high school that used to have d size boobs or really nice c cups. Since they got into highschool most joined cheerleading. About halfway through the year I noticed that a couple of the girls have gone down to at least a b cup maybe even large a cups. Is it possible that cheerleading did this, and if so if they stop will they go back to thier origonal size?