Given how long ago that default was added, it was likely to try and set a sane default to avoid exceeding the 4MB BSON max document size that existed at that time (increased in SERVER-431 to 16MB).
Our team is currently working on NODE-5906, which may include evaluating the performance implications of removing the default batch size. Assuming we observe a positive impact (
) we would consider changing this behavior in a subsequent major release.