I have literally no butt. Why am I rounder in front than I am in the behind? If my stomach was as flat as my behind I would be one happy girl.

I want a bigger butt so I can fit in jeans without having the butt of them sag. Are there any exercises that will help?

Every and all variations of squats and lunges will help your butt and legs, sometimes all the work in the world will not help especially if it's genetic, unfortunately ... but those exercises will certainly make a difference...
Google it...

Exactly.

And with lunges, do dumbbell lunges, and put your frontleg forward as much as possible (of course to the extent that you can still do the movement like you should do it). But the more you put your front leg forward, the more emphasis on the butt.

Step-ups are good too.

And hip-extensions.

And a lot of women don't know this, but back extensions are 'butt-builders' too. Many people think these are for the lower back, but the emphasis is on the butt actually.

And don't 'isolate' the butt. Do hamstrings too. You'll do that with squats and lunges already, but any form of deadlift will help too.

These are the absolute best butt builders! ;-)

Stair exercises might help!