Every pound armor is lightened beyond what the base weight of that armor type is listed as will grant you negative encumberance and increasing your carrying capacity.

Example: Robes are a base weight of 8 pounds. You can lighten them by half to 4 pounds. You could now carry an additional 4 pounds(This number will be further modified by your race) of weight before feeling the effects.

https://gswiki.play.net/Encumbrance

Look at the race encumbrance section for details on racial modifiers and a few examples.