Early satiety
When the tumor has grown large enough, it starts pushing aside other abdominal structures, including the stomach. There are many different mechanisms to control satiety, and one of them is the distension of the stomach walls. By exerting pressure to the stomach, a large tumor would send a signal to the brain that we had enough food closely after starting to eat. It can be compared to what happens when obese patients get a gastric balloon surgery. They feel satiated because an additional abdominal mass leads the brain to believe the stomach is already full.