It sounds like your gallery might have fixed pixel dimensions (rather than being responsive) and, if so, it will always be displayed at the specified dimensions on all devices and in all browsers.

There are essentially two ways to make a Juicebox gallery responsive (with its dimensions dynamically changing with the size of the user's browser window).

Scenario #1:
A Juicebox gallery will be responsive (and will dynamically scale with the size of the user's browser window) if the gallery's own dimensions and the dimensions of all parent containers are expressed as percentages. If there is a fixed value anywhere up the chain, then the gallery's size will become fixed (e.g. 100% x 100% x 800px = 800px).
Please note that when using percentage heights, you may need to implement the suggestion noted here.

Scenario #2:
You could use JavaScript to listen for a change in the size of the user's browser window and assign new dimensions to the Juicebox gallery if and when this happens.
An example of this can be found in the resizable galleries support section here.
Take a look at the source of this sample gallery in your browser to see how this might be achieved.

Both scenarios above apply equally to Juicebox-Lite and Juicebox-Pro.

Juicebox was designed to be embedded into a <div> in a web page.
Do you have an example of the issue you are having? If so, please post the URL to your page so that I can take a look and hopefully help further. Maybe if I see your gallery in your page, I will have a better understanding of your problem.
Thank you.

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