BEcause you are looking in the wrong place and the weight is needed for the postage costs .
You need to go to the aftermarket parts catalogues , Rotary, stens , Prime Line, Gates
They will list your belt by the OEM number, their number plus list the ( supposed ) belt dimensions
MY go to is always
gates catalogue .
Click on the red "vehicle Parts Search" the type in the Westwood part number in the search box on the new window.
Then when it lists the Gates belt number, click on that to get the full specifications.
This of course assumes that as it is your mower and you have owned it for some time you recorded the number of all of the belts .