Will swimming make my breast smaller

Maybe a little. An athletic frame in females produces smaller breasts than her chubby, less active counterpart. However it's a healthier shape.

If you lose weight with your swimming regimen, then a result of that weight loss might also be a reduction of fatty tissue in your breasts. Swimming also develops the muscles in your chest so that could give your breasts the appearance of a lift depending on your build. Different people get different results,

No, it will not make them smaller. It can make them firmer, and this is a good thing...