You said you are all thin... So the Object Space Reservation rule is 0% or simply not present in your applied Storage Policy, correct?
Also how many VM's/and how much RAM is being reserved in a vSwap file for each VM?
10 VM's w/4GB of RAM each will consume 80GB of vSAN capacity.
vSwap files are currently mirrored by default, and have a 100% space reservation. That's 10x4GBx2 or 80GB.
If you enable Sparse Swap, that 80GB would no longer be reserved, and space would only be consumed for 2x the amount of vSwap you need to use. (2x because of protection).
Check out this URL ( https://code.vmware.com/samples/1665/ ) for a PowerCLI script to enable sparse swap on vSAN 6.2 or higher.