It’s absolutely not possible – no – so you’re missing some data points to draw your conclusion.
If the skinny guy was consistently eating shitloads he would gain weight. What does this suggest? Probably that he wasn’t consistently eating shitloads. Maybe when he’s at home he forgets to eat (you’d be surprised how many skinny people forget meals because they don’t feel hungry). Maybe he only cooks basic food for himself? Unless you’re with him 24hrs a day you cannot know his entire diet.
Anecdotally, I lived with a super skinny dude for a few years and his diet was hilarious. I once saw him cook 75g of rice with a side of chips for his dinner. For a grand total of, what – 350 cals? For his main evening meal.
If the overweight person was only eating consistently a small amount of food but wasn’t losing weight then what does that suggest? It suggests that that person was eating more than you were seeing. Especially if you’re actually clinically overweight, eating at a deficit will cause you to drop weight quite rapidly. Again – there are studies that have concluded people vastly under estimate the amount they eat or vastly overestimate the impact of exercise on their energy expenditure:
https://pubmed.ncbi.nlm.nih.gov/1454084/
https://pubmed.ncbi.nlm.nih.gov/21178922/
Again, unless you’re with this person 24 hours a day there is no way you can know how much they are eating a day. (You’ve also not told us about that persons height or habits. Are they short? Do they sit at home all day? It’s amazing how if you are a shorter person who is sedentary how largely the chips are stacked against you.)
EDIT: Occam’s Razor applies massively here.
