The goal isn't make it so less food fits in the stomach overall, it's about the bodily response to less food and changing the perception of less food. Stay with me, you're getting closer. It's not about tightening the stomach itself, it's about reducing the intra-abdominal volume by decreasing laxity in the walls. Abdomen workouts significantly strengthen and tighten the abdominal wall, especially in previously poorly nourished individuals who were without training regimens and who classically have issues due to weak walls and subsequent pressure imbalances (diverticulosis, constipation, abdominal herniation, gi stasis, etc). That rebuilding of abdominal wall strength causes a decrease in laxity and an increase in resistance to distention -->decrease in intra abdominal volume --> increased intra abdominal pressure --> increased external pressure applied to stomach and intestines/resistance to expansion (obviously the same thing because of foundational principles of how pressure works) By adding that pressure you're going to lead to a change in the firing rates of those sensors like we talked about earlier, because they are based on stretch, which is highly influenced by pressure. So thay process is going to lead to less hunger hormone release as we established. So you'll feel fuller on more food because it takes less added pressure from the increase in the volume from the stomach to change the frequency from firing more hungry signals to firing less of them. And when you reduce the hunger hormone production, you're going to feel fuller faster and for longer. So what then happens over time is that because the stomach is being stretched less (because you're not eating as much because you feel less hungry) it is going to slowly become less stretched at baseline as it was before. Notice the feedback loop there. Over time, that process occurs, and the sensitivity of the stretch receptors will also continue to adjust along with the stomach. So now, the stomach becomes less and less stretched as baseline, so it takes less volume in a single serving before starting to stretch (but if you keep pushing it, it will go back to its previous state of being more stretched and holding more at baseline as before) and that stretch is going to shut off that ghrelin signal quicker because the sensor is detecting 100% stretch sooner. It's not "shrinking" as in it holds less, it's just changing the degree of stretch, the baseline of stretch, and the sensitivity to responding to stretching, meaning treating a lower volume as the baseline and changing the scale for fullness because of its change to sensitivity to stretching (both at the level of the tissue and the sensors). This is part of why momentum is so important weight loss regimens, it's all down stream effects, and they don't happen overnight. Hence the "regular" aspect of it. Another good example is if you do workout and you utilize phases of cutting and bulking, when you switch back to a bulk after a long cut, it's hard to get all that bulking food down even though it was easy at the top of your last bulk, because your baseline has changed.

(Again, simplified for ease of understanding)