Strange hands cramping?

[Sandwich Man]

I'm 5'8 and I really don't have very large hands. I play a jazz bass, with a jazz neck, and my hand cramps up. I went to guitar center recently upon the notion of buying a new bass and am still thinking about it. My hand, being about average to smallish for my height, has one trait that keeps it macho, a HUGE thumb. I measured it, and it's the size of my index finger. My hands are also fairly strong.

My question is, will a fatter neck eleviate the problem of hand cramping, or do you need to adjust my technique somehow for my freakishly large thumb. I played a MusicMan SUB, which had a larger neck, for an hour or so and my hand felt great, so in a way I've answered my own question, but in the interest of saving money I would like to know if someone with a large thumb like me should adjust their technique somehow.

Thanks.

edit: remeasured thumb and it's larger than my index.

[PrimusNut]

You didn't say which part of your hand was cramping up.

[Sandwich Man]

Oops. Sorry. Left hand (fretting hand) between flat of back of my hand and knuckles.

Actually I just watched some Geddy Lee and tried to mimic some of his left hand and so far so good, no cramps.

[darthplagis]

do you use 'proper' thumb @ back of neck or free thumb/claw style? both have their merits and flaws and both have different stress points on the hand which can be triggered by certain situations like position on neck, bass/strap height/angle, speed of runs etc. from where the pain is coming from i would say relax the thumb and use the fingers > thumb for the pressure rather than relying on the thumb to put pressure on the strings. also when you drop down to the E or maybe A cos you have small hands try and drop the thumb round the back of the neck a few MM (towards the G but not too far from the peak of the curve) so the fingers dont stretch too far as this could be the cause of the cramps.

the MM neck prob helped support the palm of yr hand as its a bit fatter at the back than a jazz but the width of the jazz will help with speed runs and string skipping.