Open your database file with filemaker, find the layout and enter layout mode. I expect you'll see a single fow of fields and about an inch or so of white space below it. Use your mouse to drag the horizontal line that forms the boundary between this part of your layout (called the body) up until the line cannot be dragged up any higher. If you can't get it flush with the bottom of your row of fields, check for any objects not visible in the above screen shot and remove or reposition them and try again.
That should remove the extra space shown in your screen shot.