The entire size of the message on disk (including additional trace
headers added by my MTA) is 11,374,046 while the size of the offending
line is 11,370,773. That means that the rest of the message headers and
text/plain part of the message occupy 3,273 bytes.
It occurs to me that allocating 11 MB shouldn't be a problem on any modern
system. But really, this isn't necessary; once inc(1) parses the headers
it doesn't care about the content. It could just go in a loop and
read data and write it out. All it REALLY cares about is converting
\r\n to \n.