The trick to this question is the 'less calories than you use'. You could obviously 'gain weight' by drinking several kilograms of water, but if we are talking about building tissue things get a little more complicated.
Metabolism is highly flexible and in a more general sense tied to tons of lifestyle factors. It's not uncommon for athletes to change weight rapidly in hospitals. Extremely lean athletes may put on several pounds of fat while more muscular athletes can lose pounds of muscle. Clearly there is nothing inherently anabolic or catabolic about hospital diets, but the change in behavior makes the whole thing make sense.
The same sort of nuance is needed when thinking about changing body composition. Since a normal persons BMR (amount of calories needed just to keep the body running during the day) is in the 1000-1500Cal range, it's pretty clear that activity is not the be all and end all of 'calories used'. If you suddenly change the type and quantity of calories, the body can respond by reducing calories burned through metabolism and adding fat stores to aid in future survival. This will have some adverse effects on day-to-day performance, but the body sees it as an acceptable tradeoff.
Tl;dr - A sudden reduction in calories can cause metabolic changes, so even if X amount of calories barely maintained a body weight it's possible to gain fat at a calorie amount less than X
Metabolism is highly flexible and in a more general sense tied to tons of lifestyle factors. It's not uncommon for athletes to change weight rapidly in hospitals. Extremely lean athletes may put on several pounds of fat while more muscular athletes can lose pounds of muscle. Clearly there is nothing inherently anabolic or catabolic about hospital diets, but the change in behavior makes the whole thing make sense.
The same sort of nuance is needed when thinking about changing body composition. Since a normal persons BMR (amount of calories needed just to keep the body running during the day) is in the 1000-1500Cal range, it's pretty clear that activity is not the be all and end all of 'calories used'. If you suddenly change the type and quantity of calories, the body can respond by reducing calories burned through metabolism and adding fat stores to aid in future survival. This will have some adverse effects on day-to-day performance, but the body sees it as an acceptable tradeoff.
Tl;dr - A sudden reduction in calories can cause metabolic changes, so even if X amount of calories barely maintained a body weight it's possible to gain fat at a calorie amount less than X