Hi
If your clip has the wrong aspect ratio (i.e. is not 16:9) then VP will fit it vertically into the screen area and in the left preview pane show the side gaps as chequered areas. In the right hand preview the chequered ares are shown black.
The IN/OUT markers have no connection with this, they are used simply as a method of trimming the length of the selected clip at one or each of the ends.
If you want full screen images (that is full frame images) you must use clips that are of aspect ration 16:9 (e.g. 1920 x 1080 or 1280 x 720 etc.) If your clips are showing a chequered area on each side then you will have to crop them to 16:9. setting the rectangle used for this effect to only include the image. Inevitably this will mean losing the top and bottom areas. As an alternative and if you don't mind the distortion introduced you could use the scale feature to lengthen the frame horizontally.
Nat