If you have been obese even slightly then chances are pretty high that you will have a larger glutes. In other words the size of your glutes will be not be in proportion to your body. However, it is easy to reduce the size if you know the trick.
Oh and by trick I do not mean any fad diets but exercises.
elliptical trainer: Every gym has these elliptical trainers on which you are supposed to tread. use them for at least half an hour per day and within a month you will be amazed by results.
Squats: This is a very crucial exercise and it not only reduces the size of lgutes but also boosts testosterone levels in your body. Eventually you will have a better sex and healthy life with just one exercise.