Why do I have multiple versions of Photos libraries?

The directory listing you supplied shows three libraries: one old original iPhoto library that has already been migrated to Photos format and has been unchanged in over three years, a library with a name suggesting that it was the result of converting that old iPhoto library into Photos format but with updates as recent as March 6, and another library with a name suggesting that it was created directly as a Photos library.

The migration process that I infer from the names would have resulted in the two libraries with "iPhoto" in the name sharing disk space for the original photo files by hard links for the migrated photos, but other portions of the libraries such as index files would be separate. Any photos added to the resulting library would, of course, increase its size.

We have no way for us to tell from a distance which of the .photoslibrary files you are looking at when you have Photos open and do the size check you mention, but you can check by opening Photos > Settings > General and examining Library Location. Which file name does it include?

Given the file dates in the directory list you supplied, you must have used both of the .photoslibrary files this month, but you could only have been looking at one of them at the time you check its size. You should repeat the process with the other library open.

Only you can decide whether you need both .photoslibrary libraries, or how you came to have two of them. You may need to merge them if they don't contain the same photos.

Similarly, only you can decide whether your migration of the old iPhoto library was thorough enough to make "iPhoto Library.migratedphotolibrary" unnecessary.