Datastores and freespace

I have 2 data stores. one on each hard drive with same capacity 20Gig

Data store 1 is using max capacity but data store 2 when created takes a good chunk and free space is only 12 Gig.

I this this may be due to the "partitions" legacy stuff it creates, but why?

Both are empty datastores..

Below is when i create 2nd one:

How can i get second data store to be same size as datastore 1? The only difference i see is when i create datastore 1 i don't get the above "partitions" so that's probably eating up most of the free space..