Right, because 4x = 10x. Jesus, stop being so ridiculous. Also, there's some magic trick called compression. Human readable text is especially easy to compress, basically negating all those effects. So just stick to reality, everything else is just embarrassing.
The HTML part is more than double the size of the plain text part, and
when you include all of the MIME metadata needed to set up the multipart
message, the overall size of the body is about 4x what it would have been
if you'd only sent plain text (0.5k -> 2.0k).
Granted, this is not the 10x increase that Michel predicted, but it's
easy to see how a *different* HTML message, with a lot more markup,
could certainly reach that threshold.