Where are you seeing the new size of the Library? In Info for the Library in the Finder? This size is occasionally unreliable.
Unless you are using "Optimize Mac Storage" for iCloud Photos, the library size should be roughly equal to the total size of the photos you imported plus an additional 25% of the size of the photos for working copies to speed up the browsing - thumbnails, previews, faces, plus database files to describe the albums and metadata. temporarily the size can be slightly larger, while Photos is scanning the library for duplicates.
To get a better estimate of the library size look into the Library package and look at the size of the subfolders for the originals and resources. Ctrl-click the library, then use the command "Show Package Contents". You will see a Finder window showing the internal structure of the Photos Library. Look, but don't modify anything. The largest folders should be "originals" with the originally imported image files, and "resources" with the working copies for faster browsing. What is the total sum of the sizes of these folders?
For example, on my Mac with the least storage I have enabled "Optimize Mac Storage" for iCloud Photos. This way, Photos will remove high resolution originals from the Mac and keep them only in iCloud, if the storage gets low. and
The total size of the imported originals is 220GB, but only 5GB of the originals have currently been downloaded from iCloud, all other photos and videos are only optimized available. The total of the working copies in the library is just 16GB.