Based on my experience fasting (as a huge guy) stomach size fluctuates quite a bit and is mostly conditioned on your eating habits. There have been times in my life where I could finish off a large pizza myself. These days I feel stuffed after one or two slices and a breadstick regardless of how little I’ve eaten during the day. Days I eat less I find I can eat less later. If I have breakfast (usually only ever on vacation) I can eat more later in the day before feeling full.

I agree there's a range but - arm chair doctor here - believe that a bigger abdominal cavity leaves more room for that expansion, so bigger people are able to expand their stomach to a larger size before pain and discomfort.