When I eat, I don’t eat very much. It’s not a matter of how tasty it is, I just become full quickly and the thought of eating more just becomes unpalatable.
Thus, usually, hamburgers go half-eaten, and 90 % of the fries survive. (Not that that is my typical diet–just to illustrate my point.) Sometimes it’s embarrassing. Everyone else finishes their plates and I’m left with a mound of food.
What’s up with that? Can one “train” one’s stomach to shrink to a certain size to which it will rebel at being filled any further? Does it become like that over a certain period by not eating very much? Or is it the mythical “I am full” hormone triggered by the hypothalamus (/pituitary–dunno the exact scientific location) telling you that you are “full?”
Why do I eat like a bird (yet am not thin! I weigh at least 20 pounds over ideal body weight) and get full so quickly?
Your stomach is basically a big bag of muscle, and it gets used to a particular amount of food coming in at particular times of day. You can train it to stretch out by eating large quantities of food (think Diamond Jim Brady), and you can train it to shrink up by eating small quantities of food. After you’ve trained it to stretch out, it’ll expect large quantities and it’ll growl if it isn’t “full”. And after you’ve trained it to shrink up, if you overfill it, you’ll throw it all back up (think “the end of the hunger strike”–you don’t just go right back to eating steak and potatoes, you start out slow with things like juice).
Sorry, no clue as to why 90% of your fries are left on the plate but you’re still 20 pounds overweight. 
If you are “at least 20 pounds over ideal body weight”*, this is because you don’t exercise enough to expend more calories than you take in. That can actually be a combination of type of diet (if you only eat, hamburgers, french fries and chocolate malts, you can eat less in quantity than someone else and still take in more calories) or lack of exercise.
*Everyone’s “ideal body weight” is slightly different. If you are comfortable with the size you are and your doctor isn’t pestering you about it you might just accept that this is the size that you are. But exercise is generally good for you for a number of reasons. If you typcially don’t exercise then consult a physician before you begin.