Thanks Andrew, I got an answer from my friend Enrique Nadal Soriano, and I am just copying it here:

"In FEM in general the gradient of the solution is evaluated with the

gradient of the shape functions. Usually the shape functions are

evaluated in a local space and thus its gradients. When evaluate these

gradients in the physical space (X,Y,Z) coordinates we need the Jacobian

to transformate the derivatives in the local space to the global space,

that at the end is relaed with the element size. I hope to answer your

question!"