Given you eat the same number of calories, your body expenses the same amount of calories metabolically, and you move around exactly as much before.... That's

Given you eat the same number of calories, your body expenses the same amount of calories metabolically, and you move around exactly as much before.... That's a lot of givens, given you're talking about a complex system that's evolved over the last million years to keep you from starving. But actual answer is exercise causes most people's metabolism to slow down slightly, to engage in less movements both conscious and unconscious, and their hunger to increase. Basically if your body realizes you're expending a bunch of energy it starts to conserve energy in all the other ways.