Comment 8 for bug 1707645

Oliver, from the lastlog(8) manpage:

       The lastlog file is a database which contains info on the
       last login of each user. You should not rotate it. It is a
       sparse file, so its size on the disk is usually much smaller
       than the one shown by "ls -l" (which can indicate a really
       big file if you have in passwd users with a high UID). You
       can display its real size with "ls -s".

http://manpages.ubuntu.com/manpages/focal/man8/lastlog.8.html

If you're using a filesystem that can't handle sparse files it might be best to not use lastlog.

Thanks