I run my mailinabox on a small VPS with a 20G disk. The remaining space was getting tight, so I decided to go looking for some space and I discovered that /var/log/journal/ was consuming 1.8G. This was in logs only going back to March of this year which suggests lots of people may be losing a similar amount of storage.
I didn’t need to keep all the journals so I freed-up almost all the space with
which preserves 10 days of journals. An alternative is to preserve by size with something like this