/var/log/lastlog also known as the sparse file gets written to when you log in. The size of this file is based off of the largest UID in the system. The larger the maximum UID, the larger this file is.
UID Maxium is 2101682400. Each ID on the server gets one 256 Bytes record allocated on server in /var/log/lastlog file,
That file called "sparse" file. Operating system has to create 2101682400 user records in "lastlog" file:
2101682400 * 256 / 1034 / 1024 / 1024 = 496 GB
Resulting in sparse file size 496 GB. However, since lastlog is a sparse file, its reported file size isn't always the size it consumes on the UNIX file system. The man page for lastlog explains the confusion.