Work on butt muscles. People destined to get a big butt naturally, already have a naturally big butt. So naturally, they don't think about things like this. Of course, this is about having a regular butt, not an obese butt. For the small-butt crowd, they can build butt muscles by climbing stairs, walking, running, weight-training, etc... Having too little body fat can prevent a small butt from becoming a regular butt.
Surgery or exercise. Exercising the buttocks can help or you may want to consider buttock augmentation/Brazilian butt lift. This cosmetic procedure uses liposuction to remove excess fat from unwanted areas and then it is transferred to areas that can benefit from the added volume such as the buttocks or hips.