SSL handshaking causes more traffic. It may be compressed, but small pages like the Google main search page (the heaviest hit) may show an increase in traffic, even after compression is factored in. Here's one place where HTTP vs HTTPS is discussed, and a few quotes:

"Servers that are heavy on serving a fairly small set of static pages that can easily be cached in memory suffer from a much higher overhead... SSL handshaking is the major cost of HTTPS."

http://stackoverflow.com/questions/149274/http-vs-https-perf...