This happened because the old drive has a partition of 60GB and the data was migrated with the partition as it is.
The good news is that you can simply extend your partition occupying the remaining unallocated space. Please go to Control Panel, Administrative Tools, Computer Management, Disk Management, and then right click the partition (usually named as C:) and select Extend Volume. Increase the partition size as needed.
It is recommended to back up your data before proceeding in case you modified or added new files to the SSD after migration.