User blog comment:Rappy 4187/Technical Update: May 20, 2015/@comment-3388044-20150521160714/@comment-452-20150521174917

In other words: While the HTML spec allows for more than H1 tag per page, Wikia expects there to be only one, and applies styles to the H1 tag used for the page header under that expectation, effectively meaning the specificity they use to target the page heading ".WikiaPage h1", will effect any other H1 headings on the page.

Incidentally, I had copied one of the H1 headings for my NewFilesModule, so that's oversized for me and I hadn't noticed until now, so thanks for mentioning it, SpikeToronto.