Swimming and aerobic exercise are both aerobic exercises. I think swimming is more conducive to shaping the body, and aerobics is more conducive to losing weight.

I feel that your figure is already quite standard. If you do have some fat, then aerobic exercise is more conducive to simply losing weight.

But you also mentioned that you don't want to lose your breasts at the same time. Swimming is very good for shaping girls' shoulders and breasts. Breaststroke and freestyle can shape the lines of the chest and shoulders respectively, making the lines of the body smooth and curved.

So the key is that you should know your figure correctly and how you need to change it. Girls often only pay attention to losing weight and fat, but ignore the shaping of body lines, so I suggest you take a closer look, except for the waist and thighs. Are the lines of your shoulders and chest not curved enough?