So here is a little tip that can be used for any body part but is best illustrated with these two examples, boobs and bums, or for guys shoulders and bums.
Generally if you want to make anything smaller you should move it away from the camera or alternatively if you want to make anything bigger you should move it towards the camera.
It is very, very simple, things closer to you appear bigger in real life and it is the same with photos.
So for example if you want to make your bum look smaller just push it away from the camera. Now make sure you are pushing it in the opposite direction to the camera, be careful not to push it to either side or it will look bigger.
And if you want to make your boobs look bigger, push them towards the camera, a bit of a bend at the waist actually does this very simply.
Combine both of these and you get this. Just look at the before and after photos to see the difference.
Before with no pushing either way.
After with pushing your bum out and a bit of a bend at the waist. Making your top and bottom halves more in proportion like this will also dramatically slim your waist. Hope you can see this in the pictures, as always practice away in the mirror.