[cecilpl2]

To add some numbers - your liver and muscles store about 500g or 2000 calories of glycogen. Each gram of stored glycogen is bound to 4 grams of water, for a total of roughly 2.5kg / 5lbs of weight.

When you eat in a calorie surplus, your glycogen stores are full. When you eat in a sustained deficit, you operate with very low glycogen stores.

This is why when you start a diet you often immediately lose 5 pounds in the first few days.

This is also why when you are dieting and have a huge cheat meal, you "gain 5 pounds" and then lose it again immediately upon resuming your diet. It's mostly retained water since your body converts the giant influx of calories into glycogen then burns it off over the next several days.