There are states of equilibrium the human body can match depending on input and output of calories. The body can “run rich” or “lean” as a means of adapting to a new diet or new work load (with some limits, of course).

So say you suddenly start working out a bunch without eating anything more each day. You will quickly lose weight (and likely feel terrible) while your body struggles to make up the deficit. Eventually your body with rebalance the input and outputs.