We’ve all had it before, there’s always one person in the gym who is convinced that the only way to add any muscle is to consume a massive calorie surplus, the contents of which doesn't really matter. Don’t get me wrong a surplus of calories is important but only when you’re getting these calories from nutrient dense, healthy foods. There are two reasons as to why ‘dirty bulking’ is detrimental to your muscle growth potential and overall body composition.
Just don’t get too fat. Even when ‘bulking’ don’t use this as an excuse to bombard your body with junk food.
Keep body fat within a certain range at all times.
The ideal range is different for different people, depending on insulin sensitivity and other individual factors. If you exceed this range, you'll know it. More of the weight you put on will be fat, gains will slow down, the pump you get during training will be reduced, and you just won't have that same degree of muscle fullness. It's obviously best to avoid this in the first place by eating smart and avoiding these ‘dirty bulks’ in the first place.
Remember, you can’t out train a bad diet. Get to the gym and work out, then get home and work in.