When I restore an EBS snapshot to a larger size, why does df not reflect this?

Answer

This is because the file system itself that was backed up in the EBS snapshot is still the same size it was when the snapshot was created. So, while the volume is larger, the file system itself is still the old size.  MySQL EBS uses XFS—so, to grow the file system, you would issue the following command:

xfs_growfs -d /mnt/mysql

Running the xfs_growfs command with the -d option grows the file system to the maximum possible size.