Your change in weight is a simple formula: Calories Eaten minus Calories Burned = Weight Lost/Gained.
Wrong. Humans are not simple combustion engines.
I am living evidence of this. I was at 190lbs - I'm tall, so that was "skinny". My thyroid basically quit functioning , and I gained 50lbs in about a year and half time.
Of course, at that time , I had no idea what was going on. When I first saw the 20lb gain,I went on a diet.
I was eating 1800 calories a day - which means I should have been losing 1lb every 5 days if compared to my basal metabolic rate (~2550 calories a day). This is very little food. That became my new "maintenance" calorie requirements. In order for me to lose weight that I put on, I had to go to about 1400 calories.
But even with an extreme case like that, that's only really a difference of 500 calories from BMR and that's a "major" metabolism problem. I find it hard to believe there are people out there that really are more than 50% off from their BMR.
Back to the central point of the thread, if you truly eat 1000 calories above your BMR on a daily basis, you should really put on weight fast.
1000 calories isn't that hard to do in murica, regardless of stomach size. There are a lot of very, very calorie dense foods that don't fill the stomach.
Off the top of my head, that's :
1 large milkshake from fast food place
1 large bowl of ice cream
2 pieces of pizza
2 peanut butter sandwiches
3-4 20oz full calorie sodas
I hate to say it, but gaining weight with "healthy" food isn't that easy to do. Nor is it easy to do by weightlifting alone.