Contours are generated at specific resolutions independent of the resolution of the grid. Electrostatic grids are somewhat unique in this regard as their resolution scales with the size of the molecule. Since electrostatic grids can be created from small molecules to large proteins, the resolution will necessarily change, but the number of grids samples will be remain constant.
The actual number of grid samples used can be modified by changing the Max Grid Samples preference in the Styles/Grids section of the application preferences (see chapter 12).