The ILS is made up of two signals - a Localiser beam (LLZ) for azimuth (left and right guidance) and a Glide slope (GS) beam for slope (up and down) guidance. From memory the old Cat I standard was for the LLZ beam to be a set width in terms of angle form the transmitter, which is located at the far end of the runway. The new (?) or Cat II+ standard is for the beam to be a certain width at the threshold.
What this means is that on long runways the LLZ beam is narrower (read: harder to capture & fly) than on narrow runways. Some GS beams are also steeper than the standard 3° slope as well.