I don't use WP Twenty Sixteen but it is usual for H1 to be coded into the title of WordPress themes.

I use H2 and H3 tags in the body of my posts and pages.

And, I never use an H1 title at the beginning of a post as I believe it is incorrect to do so.

I have problems with hi and h2 because in a iPhone it Loioks so bad
So I have to use everything smaller

Thank you for your question and I use H2 and it works.

Post titles are usually as large as h1 headers.. How does it look like in your theme?

I have never understood this issue. I use 20-10 theme and my titles look small. If I use H1 later as subtitle in text it looks bigger than title. So I only use H2.