Signature shows too BIG to recipients

I am using Apple Mail 4.6 and have simple text lines for my signature. Even though I have the "match text in email" box checked, the signature shows as much larger (obnoxiously large) to many recipients. I've also tried it with that box unchecked, but that didn't change anything. How can I make the signature lines match the text font and size of the message? It looks fine on my computer but arrives huge to the recipient. MAC OSX 10.6.8 (Yes, we do have plans to update some day)