I've talked about how some folks fear that lifting weights will cause people to put on too much muscle. These people have probably never lifted in their lives. If they did, they would know that it takes a LOT of work to pack on tons of muscle.
Most people overestimate the benefits that they'll get from exercising. They think that if they run a couple of laps, they've made a big dent in their fitness goals. Or worse, they think that they can indulge in that cheeseburger they've been craving.
I see this in our gym classes all the time. People do practically nothing in the cardio classes, scarcely working up a sweat. They probably figure that they've put in their time, and that that's what counts. No, it doesn't. If you consistently work out at a low level, then you'll scarcely see any results. You need to put in some real effort if you want to get real results.
I'm not saying that every workout needs to be intense. The occasional low-intensity workout can be beneficial, especially if it's an endurance exercise or part of one's recovery period. However, consistently working out at a low level is likely to yield low-level results and nothing more.