By default, Juicebox will scale down large images to fit within the gallery's image area as large as possible whilst respecting their aspect ratios and without cropping. However, small images will not be scaled up (as this may decrease their visual quality).
If you find you have space surrounding you images, then make sure that your images are large enough that they would need to be dynamically scaled down slightly by Juicbox when the gallery is displayed. This will ensure that they are displayed as large as possible without cropping or stretching within the given area.
If you still have space to the top and bottom or to the left and right of your images, then please see this FAQ:
My Juicebox gallery shows too much space above or below the main image, how do I fix this?
Juicebox-Pro users have the option to change the way that images are scaled within the gallery's image area via the imageScaleMode configuration option. Possible values are SCALE_DOWN, SCALE, FILL, STRETCH and NONE.
Short descriptions of each of these values can be found in the Main Image Options section of the Config Options page.
Please note that imageScaleMode is a Pro-only option which is not supported by Juicebox-Lite (the free version).