A very major factor is being left out. When a person begins to piss, the bladder itself has contractions that force the urine out of the body. It is like taking your fingers and pressing on a balloon. And this is off topic, but this is also the final cause that most people lose control of their bladders and wet themselves. At full capacity, eventually the brain sends the overwhelming signal for the bladder to start contracting at that point unless there is a blockage urine control is loss.
So when a person has a fairly full bladder and then gives the approval for the body to piss, the bladder has very strong contractions that empty the bladder. In other words the closer a person is to pissing on himself because of a full bladder the stronger the urine flow is going to be. A person who has to pee for the doctor and just went not too much earlier and dosn't have very much urine is not going to have a large volume flow; not enough urine to produce that "Niagra Falls" flow.
Some people have very weak contractoins or an overstreatched bladder, they have a very weak stream and sometimes can't empty the bladder completely meaning they have to go often and piss little with a weak stream. It has absolutely nothing to do with penis size.
The size of the urethra does matter some. That has already been covered. The pee hole itself I would think would be an extention of the urethra itself. i am not sure on that part of the body. Never heard any mention of that in medcial documents. Only the size of the urethra, the amount of urine in the bladder and the strength of the bladder contractions has been mentioned that I have been told about or have read.
Bottom line: It is more about the bladder than it is about the dick.