Xsan 4: Different Logical Drive Sizes

I an reinitializing a SAN during the process of upgrading from Xsan 3 to 4. I have 4 total logical drives, 2 of which are 10TB and 2 are 5TB. When I choose all 4 to make a storage pool for a new volume, it reports as 20TB usable of 30TB total. When I only choose the 2 10TB drives, it reports a total of 20TB. Why is it not able to use logical drives of different sizes now? (I had previously used all 4 to make a 30TB volume before).