A sparse file is a type of file that attempts to use file system space more efficiently when the file itself is partially empty.
This is done by writing metadata representing the empty blocks to disk instead of the actual "empty" space which makes up the block, using less disk space.
The full block size is written to disk as the actual size only when the block contains "real" (non-empty) data.
The /var/log/lastlog is a typical example of a very sparse file.
The -s option of the ls command shows the occupied space in blocks.
$ ls -lhs