Bootcamp Refuses to create boot drive because iso is too large?
In the past, an 8 GB USB would suffice to create a Bootcamp boot drive for Windows 10 (talking from experience), but now you need "a blank 16 GB or larger USB 2 flash drive," as Apple states on their Boot Camp support website. I too found out the hard way, as the process of creating a boot drive on my old 8 GB USB failed.

It's possible that the latest versions of Windows 10 (Anniversary Edition onwards) simply require more than 8 GB for their installers.