Hi, I don't usually feel full.  If I absolutely stuff myself I feel
full, but this only happens a couple times a year.  Usually I can keep
on eating well past the point where others complain about feeling
full.

I was wondering about the chemistry behind this -- what is chemically
different about me? I understand that seratonin gives people the
feeling of fullness, but I don't think I'm lacking in seratonin.

Interesting question! I doubt that there's a definitive answer, though.

The hormone leptin (which is produced by fat cells) helps to signal a
feeling of fullness after a meal. It is possible that you have a low
serum level of leptin.

Another thing that may influence one's feeling of fullness after a
meal is the presence or absence of substantial amounts of waste matter
in the colon. If you have frequent bowel movements, you may not feel
as full when you eat. I became quite aware of this after I had my
colon removed, which seems to give a permanent feeling of hunger
(apparently "colon is gone" and "colon is empty" produce the same
signals). If I were to eat until I felt full, I would probably weigh a
ton. I've just had to get used to being somewhat hungry most of the
time.

How old are you?

And is your weight OK?