I can't the exact details but IIRC there is a canonical study which tracked physical activity, calorie intake and obesity in large cohort. There was a high correlation between calorie intake and level of activity in the sample and likewise only a weak correlation between level of activity and obesity (or lack thereof). Basically, people were good at calibrating their calorie intake to their level of physical activity. EXCEPT for the sedentary people, who tended to consume calories way outside of their needs based on little or no physical activity and (not surprisingly) tended to be obese at high rates.

Our bodies seem to be good at calibrating hunger and calorie intake to maintain balance between calories in and calories out, but there is something about being completely sedentary which causes that mechanism to malfunction.