Why don't you copy it to a new name like messages.old and gzip it up. It wont cause any issues to delete it, but you will lose any messages that may be useful in trouble shooting your system.
/var/adm contains mostly logfiles and they normally grow unbounded. That means that you must make regular checks for files that are growing fast and make a copy of the information for business audits and truncate the files. The copies can then be compressed using compress or gzip, typically to less than 1/4 of their original size. Depending on your audit requirements, you may need to keep the old logs for 6 months, maybe longer. Be sure to look in /var/adm/syslog.
You should be checking daily or perhaps weekly to monitor log files. OR: you write a script to do this for you, or download a copy of logrotate:
Actually, the man page for dmesg has suggested /var/adm/messages since the early 1990's. I always thought it was an odd choice and use /var/adm/dmesg.log which seems more natural.
