I wish I was given the "go" to eat 6k calories each day! Although I'd probably blow it all on pizza and ice cream; glad RG4 appears to have been more efficient with his caloric intake.
I watched a documentary called Generation Iron...It is about bodybuilding and what they go through just to make it to the Mr. Olympia meet. Those dudes take in 9k+ calories a day and it is all low fat foods.
There are bike rides that I take that are 2000+ calorie burners according to Strava every weekend. Even though I can eat anything I want to at the age of 41 and still retain my TDF physique, I tend to snack on energy bars, and protein shakes as desert because pizza and icecream junk foods are not good fuels when you are exerting yourself for hours at a time. They sit in your stomach like an uncomfortable brick.