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.
The work done to obtain perfumes and to recite invocations is better than the work done with precipitable gases and other spirits. - Picatrix (Andalusia, ~1000.AD)