A common practice is to use non-preallocated virtual disks.
Since they only grow as needed, it makes them perfect for fast backup, overallocation and creation speed.
Since file systems are usually based on physical disks they have the tendency to use the whole area available1 in order to increase the speed2 or reliability3.
I'm searching a filesystem that does the exact opposite : try to touch the minimum blocks need by an aggressive block reuse.
I would happily trade some performance for space usage.
There is already a similar question, but it is rather general. I have very specific goal : space-efficiency.
1. Like page caching uses all the free physical memory
2. Canonical example : online defragmentation
3. Canonical example : snapshotting