Can i lose a bra cup size when i lose weight?

Sure: People get bigger breasts when the become overweight or obese (breasts contain fat, milk-producing glands, and connective tissues). Likewise, people get smaller breasts when they lose weight. There is symmetry in nature. Otherwise, a person would look rather unusual, having a skinny body but huge breasts.

Possible: but not likely since the only tissue you can loose would be the fatty tissue overlying your breast tissue. Hope this is helpful! Dr Z