That's odd.
I've just created a test gallery on my own Windows 10 PC (using JuiceboxBuiler-Pro v1.5.1) with 'Large Images' selected (at the default size of 2048 x 1536) using source images of 2048 x 1536 and, as they are already within the maximum images bounds, they are copied across to the 'large' folder as expected.
I've also tried with images both smaller and larger than the maximum image bounds and everything seems to work as it should (at least for myself in my own tests).
Unfortunately, I'm not sure what might be causing your problem. It's not a known issue and not something that I remember anyone reporting before (and I cannot replicate the problem myself).
The only thing I can think of that might be causing the problem is if you maybe have the 'large' folder open in another program (perhaps to inspect the images) and the other program has a temporary lock on the folder preventing JuiceboxBuilder-Pro access to it.
Try running JuiceboxBuilder-Pro with no other programs active (other than the usual system processes) and make sure that you save your gallery to a new empty folder.
If this does not help, then a fresh install would certainly be the next thing to try.
Please try the complete uninstall/reinstall procedure noted as #11 in this forum thread.
If this does not help, then perhaps you could provide some of your source images (maybe upload them in a zip file to a file sharing service such as Dropbox or Google Drive) so that I can try to replicate the problem using the same files that you use.
If you don't want to make your images publicly accessible, then just let me know and I'll email you and you can send me a download link privately.
Also, if you could let me know exactly what settings you use in the JuiceboxBuilder-Pro interface, then I'll hopefully have a better chance of reproducing your problem. (A screenshot or two might be the easiest way to let me see your settings.)
Please let me know how you get on.
Thank you.