We can learn some things about the stomach from competitive eaters.
Competitive eater Joey Chestnut trains his stomach to expand:
Chestnut practices by drinking up to a gallon of milk in a single sitting, which
he says trains his stomach to expand.
Competitive eater Sonya Thomas also says her stomach has been trained to expand:
My stomach is really normal size, but over the years it has been trained to
stretch quite a bit.
From Huff Post, which references Dr. David C. Metz, a published gastroenterology professor:
Our hypothesis became that [competitive eaters] have the ability to relax their
stomachs to such a degree that they can just eat and eat. While eating, the
average person's stomach may expand to hold two or three liters, he imagines,
whereas a competitive eater's may expand to hold six or seven.
There isn't a lot of research around this, and Dr. Metz's study is one of the most cited studies for competitive eating (according to the Huff Post article).
From this limited information, I can only hypothesize that if a normal person's stomach expands to about three liters and a trained competitive eater's expands to seven liters, it's possible that a stomach can lose some stretching capacity when one eats less food.
Either way, it's important to make a distinction between the size of the stomach and the capacity of the stomach. Even if gastric capacity was reduced (as cited by the OP), that may or may not have any impact on the size of the stomach.