You made the hands right? When you compare them with the regular C_Hands from GMod, the distance between each bone should be the same on your hands, too. Otherwise, they will be stretched, and that is probably the issue here. You would need to rerig the hands. Make the arms longer so the bones will have enough space between each other.
Try to scale down the mesh (only mesh, not the skeleton), and put the hands further away from each other. Also, looks like your hands are starting from the Clavicle bones. They shouldnt realy start there, but from the UpperArm bones, as you cannot realy see the mesh there.
When you look at my Nanosuit Hands, they even start waaay down at the Forearm bones. I would start the mesh somewhere around where the cursor is between the Upperarm and Forearm bones. Your hands are probably too long