Fastvue Sophos Reporter imports the Web Filtering logs from Sophos UTM. The bandwidth figures are therefore the sums of the logged file sizes for each web resource.
For example, when you browse a web page, your browser downloads all images, html files, scripts, styles and so on. Each one of these 'resources' is logged by the UTM along with its size. It is the total size of these resources that is shown in Sophos Reporter.
Keep in mind that very large downloads will not be logged until they have completely downloaded. So you will not see the bandwidth figures increase during the course of the download, but once it finishes, the Total Size values will suddenly increase.
Also keep in mind that there may be other features of the UTM consuming bandwidth such as Webserver protection (reverse proxy), SSL VPNs, Email protection and so on, where the traffic does not go through the Web Protection engine. Depending on your usage of these other features, Fastvue Sophos Reporter may be showing you a sub set of the total bandwidth going through the UTM.
Also note that it is possible to turn off the logging of accessed web pages and blocked pages in each of your UTM's filter actions. If logging is disabled, then you will not see traffic passing through that Filter Action account towards the bandwidth figures in Fastvue Sophos Reporter.
Hopefully this helps explain how bandwidth is calculated in Fastvue Sophos Reporter. If you have any questions, please let us know.