Heading tags are often really poorly used on the web. It’s easy to think of them as an easy way to set your font size, but you’d be making a mistake if you did.
I’ve posted a blog to Purple Web Marketing about heading tags and SEO, have a read.
The tl;dr version is really simple. First and foremost heading tags should be approached with SEO in mind, but there’s nothing wrong with playing around with your font styles to make sure your key text is used as headings – even if it’s not your largest text.