Please attach an image to a post (or upload an image somewhere and provide a download link) so that I can see what you are seeing.
Also, please let me know what device and browser you are using to view your gallery.

In the meantime, the following notes might help to clarify some things.

The gallery will be displayed on the web page into which it is embedded at the dimensions specified by the galleryWidth and galleryHeight configuration options in the gallery's embedding code.
In desktop browsers (where the gallery is displayed in Large Screen Mode), space is reserved for the thumbnails and the TOP area (if used) and the remainder of the space is used to display the main images.
On mobile devices (Small Screen Mode), thumbnails and main images are displayed on separate pages and the TOP area is not used so the main images take up the entire gallery area.

No matter what size the gallery is, the main images will be scaled (within the gallery's image area) according to the value of the imageScaleMode configuration option.
Please see the entry for imageScaleMode in the Main Image section of the Config Options page for short descriptions of the possible values (SCALE_DOWN, SCALE, FILL, STRETCH, NONE).

Also, if you find that you have too much space at the top and bottom (or left and right) of your main images, then please see this FAQ which might help.
My Juicebox gallery shows too much space above or below the main image, how do I fix this?