Do it again, and "fancy it up" with fancy fonts, fancy font colors, etc., (like some people like to do), and really see how it grows. Multiply by the bazillions of email messages that get sent to-and-fro on a daily basis, and you will understand how much it impacts bandwidth.
And yes, even a few html attributes can make a message larger, than what a somewhat-intricate plain-text signature does. I subscribe to tech updates from places such as cnet.com. If I receive one that uses html, and I download and open it in a non-html email program, the html tags use up more space than the actual text.
Similarly, this week, a friend emailed me the infamous "three old ladies beating up a guy, at a traffic accident" cell phone recording. The thing had been forwarded (all over the world, apparently) about a bazillion times....in friggin' .wav format!!! It was like 4MB HUGE!!. I converted it to .mpg, and it shrunk to less than 10% the original size. I then re-sent it to the person who originally sent it to me....along with a stern lesson on shrinking one's email messages...lol