You're not supposed to completely fill up the rear sight notch. What you're looking for is the top of the front sight even with the top of the rear sight, and equal amounts of daylight on each side of the front sight.
Many practical competition shooters file down the sides of the front sight in order to give them more daylight to play with.
Many practical competition shooters file down the sides of the front sight in order to give them more daylight to play with.