well, just to cover the silly things,
make sure the object is at the top of the detail section.
all objects are the same size
the height of the objects is as small as possible
the bottom of the detail section is touching the objects.
the only other thought is to reduce the font as well.
if you have done all of the above, then any remaining space is placed by the system, and is beyond your control. 5mm is a pretty big gap. I would check if something is wrapping with 'Can Grow' set or an object that is taller than the rest. If the detail section is the height of your objects in design, the preview height should be pretty close.