A properly performed tummy tuck almost always includes repair (tightening) of the fascia (gristle layer) that surrounds the rectus abdominis (6-pack) muscles, since this fascial layer is stretched with pregnancy or larger weight gains. When childbearing is complete, or weight is lost, the loose skin and stretched muscular fascial layers (diastasis recti) allow the intra-abdominal contents to protrude. (An actual hole in the fascial layer is called a hernia.) This "pooching out" is what so many patients seek to improve via tummy tuck.
Of course, each patient is unique, as is their degree of abdominal wall and skin stretch, presence of stretch marks, and amount of intra-abdominal fat (along with their organs) that protrudes, and physical examination determines which patients are the best candidates for tummy tuck.
It sounds as if your abdominal wall was (appropriately) tightened, fitting your organs, and mesenteric and omental fat, into a smaller cylindrical abdominal cavity. So while your stomach is physically the same size, it resides along with the rest of your "innards" in a tightened space, and feels full faster.
Sort of like wearing a tight corset or waist cincher and wondering why you feel full faster!
This is not a bad thing as everything works the same, but you tend to eat less, and will lose weight (or gain less rapidly) with similar exercise habits.
I hope that explanation is clear. Best wishes! Dr. Tholen