What's the deal with disk quotas? Saw that in the OT as well. Why would you measure folder size seperately for each and every user? Would it not be a lot easier to just use the disk space of a folder regardless of whomever the file belongs to?
For a group shared directory, assigning the disk space usage of files therein to one single user (ignoring the aspect of "which single user do you pick") is unfair to that user (his/her allowed maximum disk space is consumed) while everyone else is not charged for their actual usage.
This all came about to try to enforce rules to prevent one (or a few) rogue users from using up all disk space on the system for themselves, leaving no one else with any disk space available for their own usage.
That's what quotas are: per-user storage limits.
If Bob has a large file which is sitting in Alice's home directory, that counts toward's Bob's quota, not Alice's. If Bob could sneakily change the ownership to Alice, while leaving the permissions open so he could access the file, then the file counts toward Alice's quota.
(I'm the author of the linked-to article.)