Page menu being squeezed and piled up on one side when minimize the screen

Hi,
I have just noticed that whenever I minimize the screen instead of a full view, my page menu as well as blog content being squeezed to one side of the page. I thought that when you minimize a full screen, lets say I drag the right side edge of the screen to the left, it supposes to truncate or cover up that part of the content where I dragged over. Hope I make it clear enough. Thanks!