If you want to increase your waist size, the simplest way would be to eat unhealthy foods and gain fat. In terms of health and fitness though, that's not ideal. Whether you're a thin person looking to beef up or you're a frequent exerciser looking for more bulk around your waist, increase waist size by gaining muscle not fat.
Workouts Focused on Muscle
Your waist muscles include the rectus abdominis or "six pack" muscles in front and the obliques on the sides. According to the American Council on Exercise (ACE), the most effective exercises for the rectus abdominis are the bicycle crunch, the captain's chair and exercise ball crunches. For the obliques area, perform the captain's chair, the bicycle crunch and the reverse crunch. ACE recommends three to five sets of eight to 12 repetitions of each exercise, with short recovery times in between. Give yourself at least 48 hours' rest between workouts to allow your muscles to generate new tissue.
Foods for Gaining Muscle
According to Joe Stoppani, PhD and Joe Wuebben of "Muscle & Fitness," nutrition is 50 percent of the equation for gaining muscle. Eat 2 to 3 grams of carbohydrates per pound of bodyweight and 1 gram of protein per pound of bodyweight every day. ACE and "Muscle & Fitness" also recommend eating carbohydrate and protein shakes before and shortly after weight training for even more success.