Changing the gallery's dimensions should not alter the responsiveness of the gallery.
If you can upload both galleries and post URLs to them so that I can compare them side by side, I will be happy to take a look.
Please let me know what you experience to be less responsive in the 500 x 400 gallery (e.g. thumbnail navigation, image navigation, clicking on Button Bar buttons).
The space you refer to is likely due to a number of factors:
(1) The dimensions of the gallery
(2) The value given to the imageScaleMode option
(3) The aspect ratio of the main image
(4) The aspect ratio of the user's browser window
If setting the gallery's dimensions to fixed pixel values rather than percentages, then #4 above does not come into play (as the gallery's size is fixed and the gallery will not be resized dynamically to fit the available browser window space).
If setting the gallery's dimensions to percentages, then if you resize your browser window, you will likely see the space reduce in size.
Changing the imageScaleMode will change how the main image fills the available image area space.
Possible values for the imageScaleMode option are SCALE_DOWN, SCALE, FILL, STRETCH and NONE.
Please see the Main Image Options section of the 'Juicebox Configuration Guide' for descriptions of these values.
You may also wish to make sure that the imagePadding option is set to zero to minimize the amount of space around your main images.