The point, basically, is that the robots were born before FONT tags were invented, when everybody used Hx tags, that's why they give more relevancy to heading tags than font tags.
The fact that editors like FrontPage automatically insert fifty million FONT tags on each page, by default, is even more reason for the robots to give higher relevancy to the (rarer) heading tags.
Any heading tag, be it H1 or H6, will be more powerful than any FONT tag.