SOBR filling drives filling up

[blake.epperson]

we are backing up roughly 350 servers most of which contain multiple drives averaging about 2 drives that are 2 TB each. drives all together are using a little bit more than 1.25 PB of space on our storage. We built a backup SOBR repository with 3 850TB drives and we are running out of space on all of them. retention period for most of these is 30 days of backups (1 monthly, 4 weekly, and 3 a day for 7 days) Per the SOBR we are currently storing nearly 3PB of backup files. why are backups taking nearly 3 times the amount of space as the actual drives?