[tjogin]

Yes, it has been scientifically proven about a bajillion or so times that a caloric deficit causes weightloss, and a caloric surplus causes weight gain, irrespective of the source of calories.

However, eating 3000kcal of "healthy food" is a lot harder than eating the same amount of "junk food", because "junk food" has a higher caloric density -- that is the very thing that makes the food "bad".

A person can easily eat 3000kcal of junk food in a day and still not be satisfied, while a person who eats as much healthy food as they can may still fall shy of 3000kcal.