Obvious there isn't a 100% correspondance, but there is definitely a correlation. I haven't got time to find it online but there was a recent detailed study explaining that out of several body parts analysed in the research (i.e. nose, ears, feet, hands, etc), hands reflected penis size the most.
This isn't to say that there aren't people with small hands and massive knobs out there. Or that there's no such thing as a big-handed bloke with small mitts.
However, the correlation exists. Every single ex-girlfriend I had as well as my current g/f confirmed what I'm saying now. The biggest penises they had tended to coincide with the biggest pairs of hands.
Also when I was at school we did a Hand Size Championship and the guy with the biggest hands (they were absolutely disproportionate) also happened to have an absolute monster in his pants as we all always remarked in the showers.
My ex girlfriend shagged him and confirmed that his cock was from another planet.