Weight gain in a dream often symbolizes an emotional burden -- responsibilities, feelings, or obligations you've been taking on that feel heavier than they look from the outside. It can also touch on self-image, especially if the dream carries shame, which usually points to the burden itself rather than the body.
Whosoever contendeth with another & overcomes when the significators are in signs bicorporeal, gets a great victory; if overcome, looseth much; for then the good or evil is doubled. - William Lilly (1602.-1681.)