On average, mothers in the University of Alabama study had bigger waists in proportion to their hips than women without children. Some of the fat that's used for breast milk production is stored in the abdomen, explains Wood, and that's a likely spot for leftover fat to remain.
In addition, laxer muscles don't hold everything in as tightly. Some women work out religiously to get back their tight abs, and your stomach is definitely not going to do that all on its own. Furthermore, depending on your size (and your baby's) stomach muscles can get stretched out beyond their ability to bounce back fully, no matter how many sit-ups you do.