MongoDB Ops Manager backup and restore
I'm not sure which version of Ops Manager you were running, but there's been a lot of improvements in the last year. The latest docs explain the space-spacing approaches of the different storage choices:


MongoDB blockstore and AWS S3 bucket  - Each successive snapshot stores only the differences from the previous snapshot. Compression and block-level de-duplication reduce the size of snapshot data.  
File system store - uses compression only.