laplacian operator del2 multiplication by 2*dimension

Hi

Could someone explain why we need to multiply del2 by 4 in the case of 2D to get the laplacian operator results?

I know the matlab function del2 says in its help that it returns the equivalent of 0.25*result_from_del2 , but i don't see why this is so. Could someone provide the mathematical expanation of where the 0.25 is coming from?

thank you