hey! i'm really into fitness so i think i can help you
A booty can grow outwards and therefore bigger. it's a muscle area and if you put the right amount of pressure on the muscle it will grow. Most people do endless amounts of squats and lunges but they only get the tight booty, not bigger in size. To get bigger in size you have to use weight resistance and you have to keep increasing the weight you use at almost each session.
First i'll explain the logic behind using heavier weights to get a bigger butt. Let's say you go to the gym and you do normal squats using 10kg dumbbells on each side. The right weight to start with is one that allows you to complete more than 7 reps but NOT more than 12. If you're lifting a certain weight and with that weight you can't do more than 12 squats then that's the perfect weight. So let's say the 10kgs on each hand is the perfect weight for you and that you struggle at your 12th rep. You will repeat the exercise 3 times, you don't need more if you're using heavy weights. Your body has just gone in shock now, so the muscle needs to be fed right after the workout (protein shake mixed with water within 10mins after a workout is all you need). The muscle is now really tired and it thinks that it's just been under attack so now its going to focus on recovering but building itself bigger in order to avoid a future attack with the same weight. Preparing your body means that the muscles are going to grow outward (just like biceps on guys). Now the following week (remember you should only hit each muscle area once a week) you have to use 12kgs on each hand in order to shock your body again as its already prepared for the 10kgs but not for 12. Then the process of your body increasing your muscle to prepare for the following possible attack continues. This is how a muscle grows bigger and it's the same for your butt.
You can research plenty of different butt exercises on youtube and the net but only when you add hard resistance are you going to see your butt increase substantially. if you don't then you'll just keep it fit and tight thats all, maybe with a very slight increase.
Now you know why it is that even if all gyms have classes specifically for your butt almost everyone taking those classes frequently doesn't increase in size, just notices tightness.
I hope this helps. Feel free to ask whatever... i'm good in this field as it really interests me
