I think Kurios has nailed it, if that isn't too painful an image.
The central issue here, aside from "where the stream hits water in the bowl", is hydraulics. If you have a pipe (and several people on this board do), the flow rate is determined by three features: hydrostatic pressure (also known as 'head'..I kid you not..in the fluid mechanics world) length and diameter of the duct. OK, so viscosity and density of fluid are also factors, but we'll assume a standard urine specimen at normal human body temperature.

So: duct diameter='size of urethra'
length of schlong=more resistance (bad news!)
pressure built up='hydrostatic head'
amount of urine=fuel

Now, nothing to do with size of dick... I dunno. Maybe bigger dongs have proportionately bigger vents..I sure hope no one has done a study of this..if they have, it's probably been funded by the government.

We seem to have a lot of leaks from big dicks in government recently..