I've often wondering about this same issue and the above seems not to match up with conventional wisdom or science. I have never used a scale in my life with food, but since I get most of my intake from markets that labeled weight etc.. its easy to carve out through a visual estimate. Plus even if I had no health or body-weight concerns, I would have to estimate what I want to get per week or per order etc...
I find that there is a conscious amount to eat more than I care to at each meal (usually 2) but the total amount ends up being pretty low if one is to compare it to an average male of my size and build with the same goals. I suspect I have at least some digestive/absorbtion issues, but it seems like the tipping point of what turns into steady weight gain is still lower, and my body seems to be satisfied eating alot less - just not enough to corespond with those goals, so I dont know if this can be distilled entirely to inefficiency or deficiency. Although 'low' appetite might be indicating something. conversely, eating huge amounts of food w/o gain, or being constantly hungry, or visible signs of poor quality in tissue: skin, hair etc...or just obvious things like undigested matter in stools or indigestion symptoms, might be further confirmation.
