When i'm browsing my notes, I have noticed that the use of the window is not efficient, as there's a huge tab in the left hand side (between the list of sub-notebooks and the text in view mode).
Is there any method to reduce the size of this margin, to make a more efficient use of the window as it's approximately 1.5 inches in size.
This one I guess you are talking about the side margin of both editor and viewer. You can go to preference > Appearance > Editor maximum width and set 700 to give a try.
I'm not so sure, if you are not talking about this, you better provide a screenshot would be much clear.