We have found that in some cases Outlook files do not de-duplicate much at all between the numerous versions. As a result, a user may find that their space usage is increasing rapidly - every time this Outlook file is uploaded. And because an Outlook file changes very frequently, this often causes SpiderOak to back up a new version which rapidly uses up more space.
Outlook's internal file format is sometimes compressed in a way that means even though the data is similar from one version of the file to the next, a few changes at the beginning cause the compression algorithm to cascade those changes all the way through the file. This ultimately means that there is very little de-duplication can do.
As a way to combat this issue, we recommend that SpiderOak NOT archive the Outlook files directly but rather archive periodic backup copies of the Outlook files. There is an add-in for Outlook that will remind you to backup your PST file and even copy it to another folder for you if you like. You can then have SpiderOak backup this folder and operations will be much more reasonable. Please visit the below link courtesy of your friends at Microsoft: