Why does my Photos library differ in size looking in Finder vs in the "Get Info" of Photos?
I assume Photos is reporting the total size of all the pictures (and just the pictures). This is sort of confirmed by selecting all the photos in the library and then using Get Info: this shows the same size as selecting the library. 

Finder is showing the actual size of the library file on disk. This file contains all the photos but also a lot of other "meta" information about the photos, albums, places, faces, etc. that allows Photos to manage and display the pictures.