Need to know the following:
1. What exact Mac model do you have?
2. What exact Mac OS are you running?
3. For Outlook, do you have Office 2008, Office 2011, or Office 2016? I have used all 3, but am most familiar with EMail cleaning for Outlook 2011 and (currently) Outlook 2016.
4. Have you ever done any disk cleanup/maintenance/repairs, for a software perspective?
5. Are you making backups to an external device? And if so, what software are you using for that?
Now, regarding Outlook, I assume you have deleted EMails in the past. Also, you should consider deleting any you do not need. For Outlook 2011, there is a way to rebuild its database (ie, Identity "file") to permanently remove deleted EMails. (For Outlook 2016, the procedure is different).