Thanks for reposting mickc1965.
My background in metabolism comes both from 40 years of bodybuilding (no, I'm no exaggerating) as well as several years of graduate school focusing on type-II diabetes. From this, I've come to believe that it really does come down to insulin levels.
Let's define clean as low sugar/high complex carb and fiber eating (e.g. unprocessed foods). Dirty then is high sugar/low complex carb and fiber eating. These two diets differ significantly in their effects on insulin levels even when calories and macros are matched.
High chronic insulin levels favor fat storage. Dirty bulking is a high insulin diet and thus favors fat gain.
So why doesn't everybody eat clean while bulking? Think of it this way, a clean bulk is essentially the same diet as you use to get lean, only the amounts are doubled or even tripled. It is really hard to gag down so much whole unprocessed foods. These foods take longer to digest (which also results in being less insulinogenic), and take up more room in your digestive track.
Will everybody get just as fat while dirty bulking? No. The younger and leaner you are, the less fat you will gain (at least while you are still relatively lean). In addition, there are genetic differences between people that involve the microbiome, as well as peripheral insulin and adipogenic related genes that just give some guys an edge. Almost not worth discussing because there is nothing that can be done about it.
Adipocyte hyperplasia is actually an adaptive mechanism to maintain insulin sensitivity. As a fat cell gets larger, its insulin sensitivity and metabolic activity in general decreases. An individual with more small fat cells will not show signs of metabolic syndrome (i.e. prediabetes) even if he is just as fat as another individual with fewer but larger fat cells.
In the end, to do the least amount of damage while bulking, a person needs to figure out, how many calories are truly needed, how much whole unprocessed foods he can tolerate in a day, and what volume of exercise is necessary to minimized fat gain while at the same time not depleting him of his adaptive reserve (overtraining).
My adivice? Keep training volume high while bulking, including some HIIT cardio.
