Look at the bottom of /admin, there you can see total space occupied by both uploads and backups.
For reference, our backups are 6.8 GB, Uploads 2.96 GB, our instance had 1.8 Mio posts when the import was complete last Nov 13th.
If you’re getting 5.9 GB backups with “only” 150K posts then either your posts are all incredibly large, or you have a huge number/size of uploads.