Will my breast size increase with 20 pound weight gain?

That is likely but: There is never any guarantee about the distribution of pounds that are gained.

Getting Bigger: Gaining weight just to have bigger breasts sounds like insanity. Right? Putting your health at risk to have bigger breasts, or adding weight that may not increase breast size as a risk to one's health seems foolish to this old doctor. If you are that obsessed about your breast size there are breast augmentation surgeries that would be less harmful to the health than just adding weight.