While it's true that your stomach doesn't actually shrink, your stomach is a muscular organ and, like any muscle, it expands and contracts according to usage.
Which means that if you don't exercise your stomach as much, it will get "weaker" (will hold less food). And if you exercise your stomach, it will get stronger (and will hold more food).
Competitive eaters (like tiny Natsuko “Gal” Sone, a japanese singer and eating contest champ) know this - they "train" for eating events by "stretching" (working out) their stomaches by repetitively overeating. I find it interesting that competition eaters in the USA are usually super morbildy obese men, while in Japan they're often tiny men and women.
So "exercising" your stomach by overeating, allows you to put more food in the stomach. However, letting your stomach be a "couch potato" never working it very hard, it becomes less strong, less resilient and can hold less food comfortably.



