It is conceivable that it is just the availability, of course. Plenty of pet owners have found that some pets will eat as long as food is available, while some others will not. Trying to understand that seems like a good target.
Because we are offered more.
Survival instinct is to eat everything in front of you. Reduce portion sizes across the board, and people would likely eat less.
I'm open to the idea that, statistically, more people will eat more when given more. I'm... less open to the idea that we can't learn from looking at those that don't eat in excess in those situations.
I mean, people are not pets.
I've seen plenty of pets become obese when food is unlimited. You're probably affecting the "experiment" without realizing it.
The question is literally if we can find a difference between the two animals we are talking about. And that difference may be something that is addressable not at the animal level, but at the food supply level. Maybe not, of course. But a perfectly fine question to consider.
This is too simplistic and is not true for many people. There are also many people out there who, when faced with smaller portions, will order multiple servings or snack constantly between meals because they are always feeling the urge to eat (while typically having low energy with which to exercise).
Added sugar in so many processed foods should not be ignored, as per the findings of this study. Lower energy expenditure via ATP, blocking of ability to use stored fat for ATP, and leptin resistance blocking the satiety signal play a huge role in obesity. These all appear to be directly triggered by fructose (which is present in most sugars, not just HFCS).