This is terrible fucking advice. Nowhere, in any of the "eat big" crowd, including Professor X, does [i]ANYONE[/i] say the goal should be to get fat. No one EVER said to binge on candy bars or pizzas. What they said was, throwing dirty calories into a mostly clean diet might --emphasis on the might-- have a positive impact on size gains through added calories.
I'm a life time "skinny kid" and my goal was never to get fat, but I still managed to add appreciable muscle.
If you're lean, stay lean. If you really LIKE eating healthy foods, then by all means continue to do so!!!! Once you introduce bad habits to eating it takes a lot longer to kill them off, versus if you never started them to begin with.
While gaining, you should not be afraid of a hamburger or cheeseburger, or whatever, but that is a far, far, far cry from intentionally eating a lot of crap.
Dirtying up my diet did in fact help me to get to an all-time high in terms of bodyweight last year, but it was not an all-out binge. I simply ate some calorie rich items like burgers or pizza if I wanted them. I still stayed away from candy and outright shit, and I always made sure to get my protein and veggies in every day. I just wasn't anal about being super clean.
Advice to The OP--- stay eating mostly clean. If you are in need of some extra calories, first try healthy foods that are calorie rich. These include but are not limited to:
olive oil, coconut oil, etc
Whole milk probably helped me more than anything to get my size up, although I'm a traditionally carb tolerant and lactose tolerant person. 1 quart of whole milk is 600 calories. 1/2 gallon is 1500 ish.