Enter 4000 'average' users into the calculator, give them 512MB mailbox quotas and ask for a restore LUN. The LUN Requirements tab of the calculator shows that you need 11 storage groups with the database LUN size specified as 298GB whilst the transaction log LUN size is specified as 28GB. The Restore LUN size is shown as 312GB which is approximately what you'd expect given the sizes of the individual database and transaction log LUN sizes.
Now change the mailbox quotas to 1024MB. The number of storage groups changes to 21 which is no real surprise, whilst the database and transaction log LUN sizes change to 306GB and 16GB respectively. That makes sense. What can be confusing, though, is that the Restore LUN is now shown as 923GB which is clearly well above the size of any single storage group.
Why is this? Well, the calculator takes into consideration that with more and more storage groups there exists an increased probability that there will be multiple failures and therefore the calculator increases the Restore LUN size to allow for multiple simultaneous restores to occur. A nice touch I thought.