Nerhe, file size was reduced just because most likely whole PST file was infected, or at least most of it... that's why it's reduced. No point in bringing back something that is infected. That's why it's always best to make backups on regular basis. Sorry to say, but your PST file is gone for good, although you still may need some mails from it, but that's the hard part with cleaning your computer if it's already infected.
Backups people, backups... they are very big part of private security lesson.