I had a friend who always seemed like she could eat whatever she wanted and was always very skinny (5'5" and 100-120 lbs, even as an adult), but i realized recently she doesn't eat very much of anything--she eats out, fast food, and sweets, but she just gets full right away, and gets sick of certain tastes fast too, it's just how her body is. Some people are like this, some of your friends probably are restricting their food, even if not officially dieting (like not eating snacks, or sweets, or extra carbs), some may be very active, like fidgety and hyper, or play a lot of sports. You should consider the size of one's bones/frame, and amount of muscle, and overall size (like one person can be 120 and another 110 and be the same size clothes, due to differences in shape, like bust, and muscles).
Good job already cutting out soda and junk food, wow, now you may want to consider portion size and maybe try replacing any fatty protein with leaner (like turkey burger rather than hamburger) and switch some carbs (like skip bread with pasta, and have more veggies instead). Have a good breakfast, research suggests that people who eat breakfast, eat less throughout the day). Hope this helps.