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.
Sluggishness in response is due to range, or insecurity, or bad organization. - Picatrix (Andalusia, ~1000.AD)