Here's what I do to better display larger sized dresses that I have. You need to buy polyester Quilt batting. You can get it at a craft or fabric store, Target and Kmart carry it too. It usually comes in a bag but you can also buy it by the yard in a fabric store.
It looks like stuffing, but when you open it, it's a flat sheet of batting/stuffing, the size of a bedspread. Mountain Mist is a common brand. You don't need to buy it super thick, because you can fold it to get it the thickness that you need. Plus you really don't need alot of it, I'm working with an old beat-up scrap of it that I've had for years, and it's only about 18 inches x 65 inches.
1. Take your batting and fold it until it's about 2-3 inches thick. Then wrap it around the center of your mannequin and use safety pins to hold it in place. My piece is narrow, so I start it above the bust and wrap it around. You only need to go to about mid thigh. You can just cut off any extra batting.
2. Next I put a large bra on over the batting. I pull off pieces of the extra batting and stuff that into the bra cups to fill them out. The bra doesn't need to fit tight, mine is actually loose, that adds more body.
3. Next I put on an XL slip, this just smooths everything out and makes getting dresses on and off of the mannequin easier, without disturbing the batting every time.
4. Put your dress on the mannequin - you might need to squeeze her here and there to smooth out some bumps. Then you are ready to go.
My mannequin is 34-24-34 and the dress shown in the last photo is 48-38-60. You don't have to stuff the mannequin out so much that it is the exact size of the dress. Just a few inches thickness of batting give it the extra body that it needs to display larger size dresses better. Some dresses may require you to put some batting over the shoulders, if your mannequin has narrow shoulders.
Bye for now, Carol