Check out the thread on the JIRA ticket that increased the value from 4MB to 16MB. There is a sizeable debate on the ticket: https://jira.mongodb.org/browse/SERVER-431
It seems to be arbitrary why they chose 16MB, and not say 32MB. It was increased because many people needed to store documents larger than 4MB (and I presume lower than 16MB). Some people have asked in that thread to have it made configurable (like you asked), which makes sense to me. Not sure why they haven’t decided to do this.