From Wikipedia:

Most private trackers monitor how much users upload or download, and in most situations, enforce a minimum upload-to-download ratio.

How exactly can a tracker figure out how much data was uploaded and downloaded by each user?

My understanding is that a BitTorrent tracker is merely a registry of users that are currently downloading/seeding and that peers, once connected, transfer data directly. So I wouldn't think that the tracker would know anything about the amount of data transferred, much less, where it came from.