Items does not weigh a percentage, but weight like 5 units out of... whatever units have as standard (usually 30). The Design unit UI is terrible at explaining this.
The weight system works like for heroes, except you get the result as a percentage factor instead of weight in stones (or whatever is the form of measurement in the game).
Took myself some time to figure out, probably could use a slight update to the user interface to reflect how the weight works.
Sincerely
~ Kongdej