You should add the following lines to:

/usr/adm/daily

# Trim the PPP logfile
cp -p /usr/adm/ppp2.2.log /usr/adm/ppp2.2.log.old
/usr/ucb/tail -100 /usr/adm/ppp2.2.log.old > /usr/adm/ppp2.2.log

This will trim your logfile everyday. Or else your disk space
will shrink without you knowing why...