Muscle is muscle. Im not sure what you mean with "quality" but a pound of muscle gained while mega bulking is exactly the same as a pound of muscle gained while lean bulking. Only difference beeing a mega bulking results in more pounds
Well they want to get bigger and in Lee Priests case he just loves food

If coleman hadnt gotten bigger since his first olympia Cutler would now been the king. So they need to add more and more to be competitive.
A good bulk diet imo would consist of a SHITLOAD of protein, 400-500 grams/Day. Decent ammounts of very low gi carbs(lentils, beans) and good ammounts of fat with as little as possible beeing saturated. If weight gain stalls then increase fat consumption. Dont consume medium(70-90) or high(90+) gi food along with fats. But fat along with super low gi(below 50)should be ok since those foods wont create much of a insulin spike.
good foods when bulking
Chicken
Ground beef
Beans
Lentils
Any fish
Decently lean cuts of pork/beef
Brown rice(mainly ppwo since lentils and beans have a better gi for all other meals)
This all depends on your body. Some can handle lots of carbs, other turn fat while observing whole grain pasta.