How can I remove my old iPhoto library after migrating to Photos?

Once you are certain you are happy with Photos, & you are certain the Library has migrated successfully, you can delete the old iPhoto app & iPhoto Library if you wish.

The Library, as you mentioned, is 'virtual' as both are hard linked to the same files. Deleting either Library will not delete the pictures, only deleting both Libraries will do that.

The apparent size on disk is incorrect [or rather, somewhat misleading], as each Library will claim to be approximately the same size, yet they refer to the same files.

Also see - Does migrating to Apple's new Photos app actually double my storage requirements?