So the script provided by Volksman does not modify the maildirsize file. You can however modify this manually to resolve the issue. Looking over Volksman script it doesnt look like it was designed to do so (as he stated you may need to tailor it to your needs). This could also be a problem with postfix not modifying the file per what the MySql database says it should be, postifx also may not be running a query for this information so in essence "working as intended".
Personally I dont think it is a big issue to make these modifications manually, the default is set to 2gigs. I only have a couple mailboxes that will need to exceed that size. Everyone else will just need to stay organized and remove things they dont need.