Roughly
They share the same original and previews using Hard links - each database has its own support files and database files so there is overhead associated with them
And as you delete photos from Photos but not iPhoto the original and preview will not be removed from disk until that photo is delete from the iPhoto library so over time there is additional space used
After a few months if you are happy with Photos and are sure you will not be using iPhoto again then it is good to delete the photo library after archiving it just in case
LN