Honestly depends on anatomy and genetics. Weight gain may or may not affect boobs at all. It can but not everyone sees the weight chant directed in their breast size, but it would usually go down a size or two if that was the case but not extremely noticeable. Maybe not from like a D all the way to a low B or A
I can say from experience that when I was 150 lbs my boobs we’re 34DDD but when I lost weight they dropped down to a 30D (and while D sounds big, not all D’s are equal a women with 38D is a LOT bigger than 30 D) and now I’m really insecure about it because they aren’t as full or “perky” anymore