If you set your gallery's height to a fixed pixel value (such as 600px), then it will be 600px in all devices and browsers.
You can change a Juicebox gallery's height in the gallery settings window.
By default, it is set to a fixed pixel value (600px) because setting a gallery's height to be a percentage (such as 100%) can cause problems if the gallery's parent container does not have a height set via CSS. Juicebox may then not be able to determine what its own height should be (100% of what?).
In a WordPress environment, you may have little control over the gallery's parent container which is generated by your theme. It is unlikely that a height will be set for the gallery's parent container as the theme has no way of knowing what content will be placed in the page or post and WordPress pages are designed to scroll vertically if necessary. (They are not intended to dynamically rescale vertically to fit within a user's browser window height).
You could try changing your gallery's height to a percentage but whether this works OK or not may be dependent on your theme.