In a Webmaster Central hangout, someone asked if it mattered if a page used an H2 heading tag instead of an H1. “A page without an H1 title will it still rank for keywords which is in the H2 title?” ...

The belief here is that the keywords you use in H2-H6 subheadings are more heavily weighted in Google’s algorithm than words in plain text and that tags are therefore a ranking factor of their own.

Google's John Mueller said that the size of your header tags, i.e. H1s, H2s, etc, does not matter for SEO or ranking purposes. John said it might matter for users and conversions but for SEO, nope.