E-mail messages frequently contain more than just text. Your messages may include attachments ranging from pictures to spreadsheets. These attachments can be quite large, and when added up can greatly increase the size of your mailbox. It’s a good idea to remove large attachments that are no longer required to avoid wasting space on the mail server.
Warning: It is the responsibility of the user the ensure any attachments still required are saved to another location prior to deleting from Outlook.
Click Cleanup Tools > Mailbox cleanup…
Check the Find items larger than 250 kilobytes radio button, then click Find.
Click on Size to sort the search list by the size column, this will assist in locating the emails with the largest attachments.
Double click the email with the large attachment to open it > right click on the attachment > click Remove Attachment.