Tim, I agree with the overall point. However, your statement that "weight is a simple function of calories ingested as against calories expended" is simply not true. You should be able to see why if you read this:
"It is known there are marked differences in gut ecology between lean and not-lean individuals. Crucially, some classes of bacteria are more efficient at processing lipids (fats) than others, which means the same fat intake can result in radically different calories being absorbed by different individuals based on their gut ecology."
In any case, the calorific value of food is very inaccurate as a measure of how a human body will use that input - would you really expect 1000 calories of raw broccoli (say) to have the same effect as 1000 calories of HFCS or refined sugar?