UNIX filesystems support files with "holes" in them. They're contiguous blocks of NULL \0 characters which don't take up any space on disk.

Usually on most Linux distros the first user account gets a UID of 501. Since the lastlog records are indexed by UID you're seeing a file that looks like it contains that many entries, but what's really happening under the hood is that the first part of the file contains the entries for the system accounts, then holes (again these take up no real space on disk but show up as part of the file's size), then the info for UID 501. Really your file is using 4672 bytes on disk, it's just the official size is the 146292.