1500 kcal/lb is a conservative estimate, considering the caloric density of fat vs protein and that the vast majority of muscle mass is water. I've heard the number 600 kcal thrown around quite a bit on body building forums. I'm sorry I can't find the actual number anywhere (I've looked), but I think a conservative bound is perfectly valid for demonstrating a back of the napkin calculation in casual conversation.

It doesn't really matter because I've already stated that eating 7000 kcal is doable. And that (grossly exaggerated) number was based on false assumptions, something to which delluminatus has already agreed.

I think you're the one confused about calories. Even an hour of strenuous lifting burns only ~ 500 kcal. http://www.health.harvard.edu/newsweek/Calories-burned-in-30...

Ferriss claims to have spent only 4 hours in the gym throughout those 28 days, which amounts to approximately 2000 kcal in a month. Meaning he had to somehow find within himself the strength to eat one additional large pizza in a month. Even if he was in the gym for an hour a day, eating 500 extra calories per day is hardly difficult.