duzyamuthano: there are a number of factors here, none of which is related to the actual size of the cock concerned:
1. the quantity of seminal fluid (not sperm) produced;
2. the strength of the muscles behind the dick;
3. the shape of the opening (the cum slit) at the end of the penis.
1 will govern how big the load is, and i'm guessing (only guessing) that it could be influenced by things like diet, health (e.g. exercise), lifestyle (e.g. smoking). but some guys will just produce more goo than others. 2 and 3 govern how big the shoot is. i'm working on the principle that more exercise for those muscles in 2 is a good thing
and i've noticed that guys with a more open cum slit shoot further and more freely than men with a more restricted opening, which tends to make the cum ooze, dribble, or just flow.
so, as others have said, no correlation ... but a big dick shooting a big load a long way is a real treat at the end of session in my book ;D