Maggie's Farm

Whether scrawny or pudgy, challenging exercise makes a person more aware of his/her physique by pushing its limits. Yes, a good conditioning class is the best wake-up call for most people, but so is 3-5 minutes of jumping jacks or jump rope. 

The scrawny learn that they need to push themselves to build themselves up for strength and endurance, and the pudgy are forced to realize how much their pudge slows them down. Therefore they are motivated to pay more attention to their nutritional habits so they can keep up.

A physiological factor is that an hour of daily exertion tends to reduce appetite in pudgy adults. Exertion means "hard to do." Scrawny people may have to force-feed themselves somewhat.  Athletic and physically-active people tend to consume what they need, not so much what they want. 

So that is how an hour of daily cardio, calisthenics, and weights helps get people into fighting shape. The exertion itself is a minor factor but it is a motivator. Physical maintenance.