Hi Fabian,
Should I understand you rebooted not long ago after ages of uptime?
In that case it is safe to remove the file or just keep maybe 400 last lines of the file.
In any case if you were to reboot now the box it will get replaced by the current log file(which is much smaller by the way you addressed your post...)and a new log file would take place...
All the best
Victor
P.S.
If it were the case (long uptime...) you may also have in /var/adm/syslog/ OLDsyslog.log and OLDmail.log files you may want to remove also or save them first and remove...