Weight gain is quite normal and usually temporary. The physiology behind post operative weight gain is secondary to:
1)fluid retention- with any surgery there is some tissue damage and the body reacts by “hanging on to fluid” also called edema. Like when you sprain your ankle the area swells and as the tissue heals so does the ankle decrease in size. In abdominoplasty ( tummy tuck) there is a considerable amount of tissue resected, moved , streched and surgically manipulated. Thus, increased fluid retention.
2) with any physical trauma or surgery the body automatically release hormones that enhance fluid retention.
3)inflammation which is the body’s normal localized response also increases fluid retention.
4) continued trauma, infection, dependent positioning and immediate heat will only increase the swelling not help.
Thus, like you see with athletes elevate the area, apply ice, take some antiinflamatories and usually the majority of the swelling will resolve in 2-3 week with all swelling be gone in several months.