Or use something like
TreeSize that will tell you the size of a directory that includes all subdirectories and files.
If the titles between both readers aren't identical, 165 books can have a hugely wide range of file sizes between them as already said; for instance, the number of images within books and their resolution can inflate them quite a bit.
If the same identical titles are on the Kobo as on the Sony, then it's probably the size difference between kepub and epub.