The relation between number of fat cells, how much fat the body attempts to store, and the resulting urge for consuming calories is somewhat complicated, right?

The relation between number of fat cells, how much fat the body attempts to store, and the resulting urge for consuming calories is somewhat complicated, right? In other words: is fat storage purely a response to diet, or is existing excess fat tissue also a driver in eating excess calories? Because if it were purely a response to caloric input, then one could also posit that exposure to sunlight might increase visceral fat storage: the fat not stored under the skin would stil have to go somewhere.