Following weight loss, the number of fat deposits (energy stores) in the body reduce. Hormones, such as leptin, which is stored in adipose tissue and is responsible for modulating whether or not we feel hungry, feed back to the brain that these fat stores have fallen to a critical level.
The brain initiates a series of responses to overcome this. Crucially the brain tells the muscle tissues to become more efficient and burn fewer calories. However, as part of the same feedback mechanism, the body actually requires an increased number of calories to feel satiated after eating. This is because the areas of the brain involved in seeing food as a reward become more active, and those involved in resisting eating become less active. In other words, we are less able to restrain ourselves from over-eating.
Put simply, when we have lost a significant amount of weight we feel hungrier, which is then reflected in larger meal sizes and the consumption of more calories. As the rate of our energy expenditure does not increase proportionally, the net result is an excess of calories, which can all too easily translate into unwanted weight gain.