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».
А там точно есть проблема, чтоб решать её таким способом?