exec sp_MSforeachtable @command1="EXEC sp_spaceused '?'"
Looking through the results I see that the culprits are as follows:
|ObjectsInternal||3156618||2089728 KB||800296 KB||1288776 KB||656 KB|
|InstanceData||177997||4698144 KB||4697048 KB||296 KB||800 KB|
I decided to take a peek inside these tables and it looks like there are thousands of objects created every day. So what are these and do they get cleared out any time?
I found a similar question asked: User Profile Service Application SyncDB Database Size. There is an response that is marked as an answer, but it merely leads us to recommended provisioning sizes of these databases to be medium to large (100GB to 1TB). There is no explanation why. This is quite bogus as I only have 75 profiles.
I have posted a followup here: Moving the User Profile Service Application Synchronization Database