When using your email as a POP3 account outlook downloads a copy of each email received which is then stored on your computer.
This means that regardless of what you do with this email on your computer, a copy of this message is stored on the server. This is likely to fill your allocated space rather quickly unless those messages are removed after a period of time.
Before you consider removing messages from the server you should insure no other system are using those emails, it is not uncommon to see someone use their email account as POP3 from outlook and as IMAP from a phone, although this does not really make sense
To get outlook to remove the emails from the server follow the following instructions:
Step 1: open outlook and go to file >> Accounts settings (on outlook 2013), for older versions, go to tools >> account settings
Step2: Double click on the account you want to change
Step 3: Click on the “More Settings” button
Step 4: Tick the option “Leave a copy of messages on the server” AND tick “Remove from the server after….”. Messages will only be removed X days after they have been downloaded for the first time.
If you also download mail from another device, 14 days should give you plainty of time to also download them on the other device.