Caucasian people are really mixed, if you except some very insulated populations like Islanders or residents in some smalltowns in Sardinia, Italy.
In those villages - and maybe other few places - scientists researching about DNA-mapping, genetics etc. have made detailed studies about genetically transmitted diseases. An as it costs a lot of money they didn't care about males endowements.
So the chance to get a 'big one' is the same all over the 'caucasian' area!