The Finder in macOS 10.15 does not instantly update the file size for a Photos Library. It may take more than a month before you will see a change of the size. What does the "Date Modified" show for the library in the Finder? Is it older than the time you last used Photos? My library is showing "Last modified February 15, 2020", even if I have been importing photos to it every day.
If you deleted the photos in Photos and emptied the Recently Deleted album, and the status bar below "All Photos" is showing a smaller number of photos and videos than before, the library will be smaller than before. Look at the size shown in the Info for "All Photos". This size should have decreased by now. To see the Info select "All Photos" in the sidebar in Photos and enter the key combination ⌘I .
There is one exception. If you are using iCloud Photos and "Optimise Storage" you do not have much control over the size of the local copy of your Photos Library. Photos will download photos from iCloud, whenever you need to access a photo and purge photos you have not used in a long time, if you need to free storage. The size of the library will change dynamically, independent of the number of photos you are deleting.