A wizard neck is definately thin but it is also deceptively wide across the fingerboard, compared to some of my Korean accoustics which are just the opposite, fat from top to bottom but narrow across the fretboard. If you're accustomed to the latter it may be that to play the same chords on a wizard requires different positioning of fingers, ie. farther apart, and less to grab hold of causing you to pinch the neck rather than the neck supporting your grip. Personally, I play a completely different style on my accoustics and even on my L.P. than I do on my RG's, I use very little of the back side of the neck on my RG's compared to a grabbing a handful of chord technique on my accoustics.
On a side note I might suggest warming up your hands and stretching out first, and also drink fluids that do not cause dehydration, as cramping is a sign of dehydration.
hope this helps!!