When most people dress up as Santa or another fat character, they usually stuff a pillow under the stomach part of the costume. A more realistic approach is to make a fat suit. The process is very straightforward if all you need is the tummy portion. The suit can be lightweight or heavy -- depending on the stuffing you use.
Purchase two sweatshirts -- one that fits and another that is one size larger.
Cut the sleeves off the larger sweatshirt, then pull it over the fitting one.
Sew the bottom hems of both sweatshirts together. Also sew the sides together, from armpit to hem on both sides -- this prevents the stuffing from shifting to the back.
Stuff the belly area between the sweatshirts with cotton batting up to the pectoral region. You can also use plastic beads for more realistic movement -- the kind kids use to make necklaces, available in most craft stores. Using beads, however, will result in a much heavier shirt.
Sew the fronts of the two shirts together along the pectoral lines.
Stuff the pecs, letting them droop slightly past the sewn lines.
Sew the sweatshirts closed at the neck and shoulders.
Wear this universal fat suit under any costume.