In order to gain more muscle mass, at some point you need to be in a caloric surplus. You can't make something out of nothing - your body needs the extra resources to make itself bigger.

You do know that your metabolism can pull energy out of your fat storages, right? And that metabolism is extremely flexible and adaptable, to be sure? Your body certainly isn't pulling out a calculator every night before you go to sleep to determine whether you've eaten in excess or not for the day, and then deciding to build muscle or not based on that alone. That's ridiculously simplistic, and wrong. It's a multitude of processes working constantly, and factors like exercise, protein intake or adaptation to ketosis, just to name a few, are of the utmost importance. Some people tend to think of the human metabolism as a calorimeter, when it couldn't be farther from it.