This article
https://support.apple.com/en-ie/HT204478
has details on what is moved and how it is arranged. Remember Aperture was a pro-level, $80 app. Photos is a freebie aimed at the family snapper.
There is a space hit but it's not double. Both libraries point to the same set of originals on the disk. Mind you, that also means if you delete a photo in one it's not removed from the disk until you remove it from the other too. If you do migrate, best to move the Aperture library off to an external disk. That will break the link between them.