Does copying a migrated iPhotos library use extra space, given that iPhotos and Photos Libraries are hardlinked?
The short answer is, of course, yes, making a copy of a library makes a real copy including new copies of the underlying photos. 

A neat test to show this and to identify which libraries are linked is given by nohillside in How can I verify that my Photos and iPhoto library are hard linking to the same disk data?: compare a photo from each library using 

ls -li /path/from/old/app.jpg /path/from/new/app.jpg


This will show the inode number at the beginning of each line of output. If the files are linked, they will have the same inode.