Thanks for pointing this out. I've actually just edited my post before seeing your answer, as I realised after posting that there is a difference (it's easier to see in the example as both headings start with "Heading" so the fact that the words are not the same length stands out).

Having edited to fix my misunderstanding, I still prefer the more pronounced difference in sizes that shows in the edit preview, but my main hope is that the preview and final post can use the same sizes.

I was writing an answer based on the original question and this answer while you were editing. ArtOfCode‭, please see the suggestions in my answer. I, too, could barely tell there was a difference, and only because the word "heading" is in all of them. The difference between 2 and 3 is huge, though, so there's room to bump 2 down a bit without crowding 3.

I'd be equally happy with enlarging 1 or diminishing 2. At the moment, I've used 1 and 3 in one of my other posts in order to avoid 2 altogether, which is subverting the semantic intention and makes me feel bad about accessibility.

Maybe consider using 2 and 3, skipping 1 entirely? If the usual semantics are that a page had one h1 (which is the title of the question), then using 2 and 3 in the body of a post doesn't seem wrong to me. A little odd to start with ##, but what the reader (or screen-reader) will see should still make sense, right? (Just thinking out loud about workarounds.)

I like that. I've just updated my other post and it still reads fine, and now I feel better about screen readers too - thank you.