Sorry for taking so long to respond.
After reading your last post, I agree that the changes you made should not affect the quota report or the maildirsize file.
Postfix should be taking care of this file every time a piece of mail is added or removed from the account.
I wish I could be more helpful in solving this issue, however, at this point you have me scratching my head.
If I can think of a possible solution, I will be sure to post.