From rusty memory, our digestive system is approximately 30 feet (10 meters) in length. If we consider our digestive system to be a conveyor belt we can mentally divide it's length by the number of meals we have each week. Everything we eat and drink goes onto the conveyor to be processed over a number of days, eventually excreted at the other end. All of it weighs something. With steady meals, the weight of the food/drink on the conveyor - which contributes to our total weight - remains steady. (We never notice this.)
With fasting we reduce the amount going in, so the conveyor gets steadily lighter, contributing less to our total body weight. With a sufficiently lengthy full fast, the belt empties almost completely and its contribution to our total weight goes to zero. Our total weight goes down. (Yes, we definitely notice and celebrate this!)
So when we end a fast, we are again loading food items onto our conveyor. Its weight and subsequent contribution to our total body weight goes up. This happens much too quickly to be the result of fat being re-deposited. Ta da! Rapid weight regain, explained! (We unfortunately notice and lament this.)
Remember that weight loss mentioned at the top? Much of that weight was only the reduction in conveyor weight, not actual fat loss. How do I know? Long term numbers can expose trends that single numbers cannot. From these, the difference between my steady weight before and my steady weight after happens to match the result of predicted fat loss based on TDEE over the number of days of the fast.