This question has been asked before here a Ask Different. One possible answer is the disk (drive) where the software is being saved is actually too small. On your model Mac, the Boot Camp Assistant creates a temporary ExFAT formatted partition where both the Windows 10 ISO files are copied and the downloaded Windows support software files are saved. The Windows ISO file can vary in size. The determining factors is the version, edition and language. Also, the size of the Windows support software depends on the model/year of the Mac. Evidently, certain combinations of size factors can lead to space errors. So, while most users probably are not having problems using the Boot Camp Assistant to install Windows, there are a few with space issues.
Older Macs use a FAT32 formatted flash drive to install Windows. Certain Windows 10 ISO files contain files which to larger to be stored on FAT32 formatted flash drives. When this occurs, the Boot Camp Assistant will produce a space related error message.
Users also have reported that Time Machine has limited the space available to install Windows. See the question Bootcamp: The startup disk does not have enough space to be partitioned.
Finally, users have reported cases where they are able to use either the Disk Utility application or the
diskutil command to shrink the macOS partition by the desired size and create space for Windows to be installed, but can not get the Boot Camp Assistant to do the same. For example see: Mojave bootcamp assistant limits partition size.
If you are having space problems, then you should first make sure you have installed the latest version of macOS that is compatible your Mac. This includes installing all available applicable updates. If you are still having space problems, then consider using the following steps to install Windows.
- Use the Boot Camp Assistant to download the Windows support software. In this case, you are prompted where to store the files to be downloaded.
- Use the Disk Utility application or the
diskutil command to create space for Windows.
- ExFat format and 16 GB or larger flash drive and copy the files from the Windows ISO and Windows support software download.
- Boot from the flash drive and install Windows. Basically, you are installing Windows the same way as anyone who owns a PC.
An example of one user who recently successfully installed Windows 10 using this method can be found at the answer to the question Mojave bootcamp assistant limits partition size.