The SWA is a pure web proxy so it does not have any understanding of traffic based in bytes. It only accepts or rejects the request based on your policy.
For example,
lets say you make a request for a 100 meg video file, the SWA grants the request and returns the file to the browser. You get board after 20 seconds of the 5 min video and close the tab. the SWA has no understanding you stopped the rest of the video. All it knows is that user X made a request for a 100 meg file.
The SWA and UTM use time based quotas (requires HTTPS Scanning / Decrypt and scan) so you could limit a user to 30 mins of facebook or similar.
If you need to get more granular and police policy with exact byte counters I believe the XG firewall allows to you create byte based policy. as the XG is an actual router it would understand that only X bytes were transferred even tho the file headers say its 100 megs.
