Is there not some clever workaround?
It might be possible to change the size of the hit areas using CSS but it I expect it would not be as easy as it might seem at first glance. I have tried to reduce the size of the hit areas by modifying the height and width of the .jb-navigation .jbn-nav-touch-area contatiners and then applying suitable margin values to compensate for the new dimensions but this has the unwanted knock-on effect of shifting the navigation arrows from their correct positions.
Also, Juicebox takes into consideration other factors such as the position of the caption area (whether or not it is displayed on the image overlay) and whether or not there are links in the image titles or captions (which requires that the caption area is stacked on top of the hit areas, effectively reducing their size).
Juicebox was not designed to allow the hit areas to be resized or moved and there seems to be no quick and easy solution to do so manually.
If you were to set imageClickMode="NONE", you could perhaps set showNavButtons="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Button Bar' section) to add 'previous' and 'next' navigation buttons to the Button Bar.
This gives users a further method to navigate between images (and the function of the left and right arrows should be fairly self-explanatory).