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?

It's not folder size that you measure, but a user's usage: how many blocks are occupied by files belonging to a certain user, no matter where they are.

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.

Because, in the early days of Unix systems actually being used as multiple, simultaneous, user systems, you might have one group of users collaborating on a project, and they would have a shared directory (via the 'group' owner) where they would store shared items. Each user would create various files, and each file's space consumption was charged to that user, but the shared directory might contain multiple files each owned by different users (but all owned by the shared 'group' identifier, so the group could access them).

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.

One reason why Unix quotas are generally not maintained and imposed by path is that it's a lot easier to update quotas as things are created, deleted, modified, and so on if the only thing that matters for who gets charged is some attribute of the inode, which you always have available. This was especially the case in the 1980s (when UCB added disk quotas), because that was before kernels tracked name to inode associations in RAM the way they generally do today. (But even today things like hardlinks raise questions.)

(I'm the author of the linked-to article.)