For PrestaShop 1.7.
To have the Volume changed automatically when you select another Width in FO you need to create each combination at a time in BO edit product page > Combinations tab.
For example:
Select Width: 50, Volume: 500, click GENERATE.
Select Width: 90, Volume: 900, click GENERATE.
Otherwise, if you select all the values and click GENERATE once, it will create all the possible combinations and you'll end up with all the volumes available for each of the widths.