I've found that RTFs are much bigger than LRFs tend to be as well. That being said, running them through WordPad always reduced their size drastically, but one of the things that WordPad strips off is the Author/Title metadata. That's one of the things that moved me into LRFs, actually.
Even so, I appreciate your making a tool like this, boradicus. An excellent "entry" to MobileRead!
And welcome, by the way.