When an <h1> is inside an element like <section>, it is treated like a header of that section and is styled relative to the section rather than the whole page. However, if any of the other headers are used in the same way, they are only styled relative to the page.
If, for example, an <h1> and <h2> are within an <hgroup> within a <section>, the <h1> will be styled relative to the section while the <h2> is styled relative to the page. This results in the <h1> actually being smaller than the <h2>, even though they are actually part of the same header and should maintain their relative size difference.
A minimal testcase is included in the URL field.
I have also filed a Mozilla bug on the matter:
Created attachment 1159 [details]
This is a more expanded testcase that demonstrates exhaustive embedding of all <hN> sizes.
This bug was cloned to create bug 17894 as part of operation convergence.
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Rationale: accepted WHATWG change for HTML5.1; re-open if you'd like to see it applied to HTML5 CR.
More details at https://www.w3.org/Bugs/Public/show_bug.cgi?id=17894