Mmap seems specific to POSIX according to this post[1], so it's not a portable way of using memory mapped files. Since there are already differences between platform capabilities in golang there really shouldn't need to be an artificial limit to how large of a chunk you can read in one operation. In 20 years I'm sure 1 GiB will be considered small.

1: https://stackoverflow.com/questions/52589949/undefined-sysca...