Why is /var/log/lastlog breaking backup software and growing to a huge size in a matter of days?

/var/log/lastlog is a sparse file. Sparse files look like large files but actually contain big holes in it. The lastlog file is not consuming the "visible" size on the storage. For instance, if you execute:

$ ls -lh /var/log/lastlog
-r-------- 1 root root 19M May 6 11:07 /var/log/lastlog

The file size appears to be 19M. However, if you add the ...