Eating anything doesn't mean you'll be destined to gain weight. It's about eating properly and eating the right foods at the right time. People think that my lanky friend 'Tom' has a super-fast metabolism. Whenever they see him eat, he's usually eating crap (ex. Quesadillas, buffalo wings, french fries, etc) and he doesn't put on weight. The fact of the matter is, the only positive attribution to his lack of fat retention is that he's young and gets good sleep (high GH levels). He also doesn't have a good workout program. He'll walk into the gym and do one or maybe two body parts a day and in between sets he likes to talk and text message. After the workout is done, he'll have his no carb protein shake and usually nap until dinner is served. Dinner is full of calories, but the protein is very hard to digest and the carbohydrates are mainly from scalloped potatoes. What good is doing a workout that doesn't stimulate growth and not taking advantage of loading proper macronutrients during that highly anabolic window of time post-training? What good is eating empty calories? What good is eating whatever comes your way and not worrying about getting what's good for you? You're probably not gaining weight because your overtraining and you're not feeding your body with what it needs; Only giving it what your 'pleasure-seeking' brain craves. Your metabolic rate may be good enough to expend crappy calories through thermic mechanisms, but you need to get in synch here.
Your chin is probably caused by excess water retention from taking in too much sodium from the crap you eat and not taking in enough potassium from good, wholesome foods such as fruits, vegetables, and whole grains. Sword burn mentioned some good techniques to get rid of some of that water, so use that in conjunction with a diet low in sodium rich foods.
Take a look at your life and revamp everything you're doing to truly live a lifestyle dedicated to healthy living and the intrinsic benefits will extrovert themselves and maybe you'll even shrink that chin of yours. If not, there's no shame in minor cosmetic surgery. Consult a good plastic surgeon for some submental liposuction.
