What you are seeing is almost certainly caused by the use of "balloons"
being enabled for comments and tracked changes (Tools | Options | Track
Changes). If the document contains even a single comment, then when you are
in Final Showing Markup view, a large margin will be allowed for these
balloons. If you switch to Final view, then you will not see the balloons or
the large margin. If you don't intend to be tracking changes and don't want
any comments, click beside Accept Change on the Reviewing toolbar and choose
Accept All Changes in Document; click beside Reject Change/Delete Comment
and choose Delete All Comments in Document. Then make sure that Track
Changes is turned off.