How much space are my pictures/videos really taking up?

Your photos are taking up the the amount of storage for the Photos Library (372.38GB) plus the amount of storage for the thumbnails and other working copies in the iPhoto Library.

Most of the storage for the original image files and some of the edited versions will be shared by your iPhoto Library and the Photos Library. when you migrated the iPhoto Library to Photos the new original image files have been created as hard links to the original image files in the photos Library. This way, the new files are sharing the storage with the items in the iPhoto Library.

I would move the iPhoto Library to an external drive to keep it as a backup, if you do no longer use it. Then you will be able to see the actual size of the items in your Pictures folder. The Finder cannot do the arithmetic for hard links correctly, because they are independent files, very different from aliases.

The shared storage is explained here:

• Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
• https://sixcolors.com/post/2015/02/the-hard-link-between-photos-and-iphoto/

the photo library is a bundle of image files and databases, the databases are describing the relations between the photos, the edited versions, the metadata like titles, the albums and projects. you have to move it as an entity, or it will not work.