Of course no two persons are exactly the same, so your statement about gain/loss is true no matter what. However, I just want to say that a wide margin is only about ~10%, or two tablespoons of peanut butter for a typical "doing nothing" metabolism. This is assuming roughly the same physique and exercise pattern [1]. People differ, but in my experience the influence of genes is overestimated quite often.

Of course, top athletes can burn through 2-3 times what a desk-worker might burn through. And I think the small number I mentioned above also tells that you can slowly but steadily put on quite some weight, by eating every day just one slice of bread more than you need.

[1] https://examine.com/faq/does-metabolism-vary-between-two-peo...