someone correct me if I'm wrong...but from what I know, if you were to do a bulk with clean food and cardio, you can get some mighty gains while still staying quite lean
most people on a bulk just eat shit for the calories and never do any cardio thinking it's going to take away their muscle gains and then complain about that chubby look or whatever you want to call it
You can in fact get leaner while you get bigger (most commonly seen with those that aren't starting with much on them)...remember, the more muscle you put on, the more calories you need, the faster your metabolism, etc...add moderate cardio to this along with a CLEAN diet and I don't see why you would have much to worry about in the way of "getting fat"
specially if you're 20yrs old and under
Now to get that crazy ass pro bodybuilder definition you'd stil have to go on a cut...not saying you're going to get shredded even when you go on a bulk if you do it "the right way" or whatever...
hopefully that made some sense, and again, someone correct me wherever I may be off base