For me the answer is simple. If I eat 2500 calories a day of vegetables, nuts, meat, eggs and fruit I have trouble finishing it all. If I eat 2500 calories with grains and sugar included I'm starving at the end of the day and it takes every ounce of will power not to eat 3000 calories. It might well be calories in, calories out but what I eat makes it dramatically harder or easier to regulate the calories in.
This is true to an extent for me as well, but nuts are extremely calorie dense. I think I could easily eat more than 3k calories of nuts in a day if just ate as many as I wanted.
