My initial guess would be the same too. There is around 20 bytes of difference between the windows and the OSX version of the libc.a file(at arm-none-eabi/lib/) and i think this may be one the reason for the difference between the file sizes built. I will dig into this more and will try to find a more concrete reason as to why this may be happening.
Can you let me know the difference that you are seeing when you build using the 2 systems?
The libc size in the "flash" column printed on the build console is 49715 for libc under macOS and 49747 under windows, so that's a difference of 32. You can also observe a similar (not exact, less than 20) for an example such as the scan project.