Outlook 2000

I have 2 question on Microsoft Outlook.
1) I noticed that when I deleted a large number of mails, the size of pst files does not change. The problem is I found out that some of my users have their pst file size gets bigger, sometimes to nearly 2 GB. How can I sole this problem and reduce the size.
2) Say that I connect to the maik server with the MS Outlook via POP3, this means that mails are left on the server, right? But some mails in inbox still have their attachment files and I can open them. The question is where are those attachments located on the HD, and if I saved them manually, will this reduce the size of the pst file?
Thanks