Just some random thoughts:

  * the time overhead on repeated connections is significantly higher for HTTPS connections than HTTP
  * you can have multiple simultaneous connections at once to get multiple small files compared with just one for a big file (though this gets limited in a browser, and can look like a DOS attack)
  * look at [HTTP pipelining](http://en.wikipedia.org/wiki/HTTP_pipelining); apparently no one uses it but newer technologies such as SPDY do similar things
  * maybe there's a way of expressing it in terms of proportion of size of HTTP headers to size of payload