Of course it's true - and I speak as someone on the portly side myself, before anyone takes offence. For me, it was more lack of exercise and eating the wrong things rather than eating too much, but since eating more sensibly and having a daily 25 minute walk to work rather than a 5 minute drive, I've lost over a stone.
I haven't dieted, I'm not going for any 5 mile runs, and I detest gyms (they take more pounds out of your wallet than off your waist), so it doesn't take that much. Willpower is the most important thing.