All surface patterns (and cut patterns) in Revit are displayed with a model line weight of 1. If you want to make them thinner, you’ll have to go into the Line Weight and change the settings for that first line weight.
Of course that will also change anything else that’s set to that line weight. If you are setting up custom line weights, it’s a good idea to reserve 1 for surface patterns and 2 for ceiling patterns, making 3 your smallest general-use lineweight.
Another way to lighten only a particular surface pattern is to change its color from black to grey. The color setting is specific to the material, so this method won’t change other objects unexpectedly.