In general more wraps of copper around a coil gives higher resistance and higher output in milivolts. As the resistance and output increases, so does the proportion of bass to treble response. So pickups with higher resistance readings will often be darker sounding, while lower resistance readings will be brighter.
If your goal is to brighten a guitar, using a 1 meg pot for the tone does very little. However using a 1 meg pot for volume gives you more treble and dynamic range. I find that a 500k pot sometimes robs too much treble and dynamics from powerful humbuckers, so I use 1 meg pot. So a lower value pot not only reduces the treble, but it also smooths out the dynamic range of the pickup.
My rule of thumb for pot values:
< 6K resistance (lipstick tubes) = 100K volume pot.
> 7-9K resistance (vintage single coils, vintage humbuckers) = 250k volume pot for singles, 500k volume for humbuckers.
> 10-12K resistance (hot single coils, medium output humbuckers) = 500k volume pot.
> 13-20K resistance (hot humbuckers) = 1 meg volume.