Message size limit set to 200MB?

In the main.cf file, the message_size_limit is set to 204800000.

Yep, that's correct, you are seeing 204,800,000. bytes

Isn't this a zero too many? Most servers don't accept messages above 20MB, so why is this set to 200?

This number is the same on a test server with newly re-built mail server.