The message size limit was reduced in c8728eab from the postfix
default [1] of 10,240,000B = 10,000kiB = ~10MiB to only
1,048,576B = 1MiB. And the documentation claims that this would be 10MiB
instead of 1MiB.
Restore the old behaviour as default and fix the documentation as well.
[1]: http://www.postfix.org/postconf.5.html