Squats. Squats with weights. Lunges. More squats. I'm personally a huge fan of kettleball swings for a full body move that focuses on your butt and can get some cardio in at the same time. I know you said without using anything, but weights increase the challenge so your muscles are working harder.

A bigger butt without the use of weights .... Idk if that's going to happen. Especially not fast.

Eat maccas until your hearts content (or struggling), should speed up the big butt process. 

Seriously though why hasn't anyone said glute raises? Not sure how to do them then pls ask.