[vacri]

If you eat a lot and then go on a diet, remember that for the first little while a part of your weight loss is simply a reduction of gut content. If you go back to your old dietary volume, part of the weight you regain is simply the gut 'filling up' again.

Of course, it also depends on the kind of foods you eat and how slowly they move through the gut, but it's just something to keep in mind when considering weight changes around dietary boundaries.

[Expez]

Another effect leading to weight fluctuations upon starting / ending a diet is the body's usage of glycogen stores as an energy source. For every gram of carbs the body stores as glycogen it stores around 4g of water along with it. Thus, when you burn through all the glycogen in your liver and muscles you will lose a lot of water weight, several kilos in fact. This weight will also be regained when you start eating normally again.