Combination of many things
size of the urethra
pressure built up
amount of urine
bladder tone and where stream hits water in the bowl
but nothing to do with the size of the dick.
Yep, that sounds about right to me, but for one thing. If you look at the hydraulics of the urine going through the urethra, a
shorter urethra would offer less resistance to the flow, and so the greatest flow (those other things being equal) would be through the
smallest dick. But I suspect that the actual constraining factor is the
minimum urethral diameter, whether at the sphincter, the prostate or the meatus (piss slit).
Another factor affecting the acoustics might be whether the formation of the final nozzle, whether meatus or overhanging foreskin, shapes the flow into a narrow stream that quietly pierces the surface of the water in the bowl, or into a broad spray that creates a lot of noisy splashing.
This is obviously a matter that large numbers of taxpayer dollars must be diverted into studying in some of the world's finer research establishments! (It would distract them from inventing more WMD, anyway.)