you are probably using outlook in caching mode.
the .pst file usually won't report the message, it is your exchange server.
for .pst file size see herehttp://support.microsoft.com/kb/830336
to turn of caching mode and save the emails to your .pst only
in outlook go Tools > Account Settings...
double click your exchange account
take check out of Use Cached Exchange Mode
This will download the messages to your pst and remove them from the exchange server.
Otherwise increase your message store size on your exchange server.
If it is actually due to .pst size you will probably want to backup both your .pst file and your archive.pst file to save all your emails