I think that i can help here.......
The size of a woman's breasts is in proportion to the size of her shoulders.....SO a woman with large breasts (apparently) really doesn't have large breasts, but small shoulders.(sqad tries not to laugh).NOW a woman with little breasts doesn't have little breasts but big shoulders, the breasts only appear little because of the large shoulders.
NOW swimmers have large shoulders, so their breasts although normal or large, look small.
Now another factor is that on entering for a swimming event, bras are not allowed under the swimming costume, hence the breasts seem to be insignificant.
Got it?