I noticed when I was looking at a series that it said in the detail page in sonarr that it’s taking 1.2TB of space on my NAS, out of ~16TB series. Since it only has 50 episodes, where would the rest of my 200+ series go? An extradimensional space inside my harddisks? 
Then I checked a few random other series, and I saw that the total size of a show reported by sonarr is vastly exaggerated in most cases. However, I looked at a new series I just added, and there the size was repored correctly.
I have the impression that quality upgrades are causing this, because the new show didn’t have that happen yet (e.g. currently downloaded in hdtv, waiting for blurays which obviously aren’t available yet).
I checked further, and if I hover over the quality badge of individual episodes, the size for those is reported correctly. It’s only the total count which is wrong.
The first show I spoke about has had quite some upgrades for each episode, as I really really like it and set it to a quality profile of “keep upgrading until you hit 1080p bluray, I don’t care if it’s just a 720p hdtv to 1080p hdtv upgrade within hours of each other”. This would explain why sonarr says it’s a whopping 5 times bigger than what’s actually on disk.
I’m on develop, sonarr is running on a synology w/ mono 3.10. I’m not really sure what I can do to help you guys find the cause? Would debug logs even show anything? sonarr has been running smoothly for so long, and I can’t recall I ever consciously looked at the disk space reported before. This one caught my eye because it said “1.2 something” which just couldn’t be right.