We've all sat in a dark movie theater munching greasy, butter-flavored popcorn and wondering how many inches it will put on the waistline. And a steady diet of it could do just that, since a large popcorn (32 ounces) from your local theater comes out at 4,200 calories and 290 grams of fat—and that's sans the extra butter. Does this mean you have to starve your way right through the credits? Not at all, says Ann Margaret Kane, R.D., a nutritionist in West Caldwell, New Jersey. "There are so many low-fat, low-calorie alternatives out there, you can basically get a skinny version of…