It's just that the semantics get read and presented in a different way. If the person can't see, then formatting can be either stripped our actually spoken out "heading one". So headings are perhaps less of a problem that other things like tables. As sighted people we deal with font size / spacing as being synonymous with the information. For screen readers, they have to extract the HTML and try to communicate it in some other way [or not]... so there's an emphasis on ensuring your HTML syntax conforms to what you want to communicate and use style sheets to handle the size / shape / colour.

@Mokubai Yeah I don't honestly mind the font size myself. I've developed a bit of a chip on my shoulder about sighted people making things harder for visually impaired people WRT HTML. Coming from the UK I've noticed a trend toward more garish font sizes expense of aesthetics. UK government are a good example (page picked at random). Over time I've adjusted and have come to quite like the direct clarity of it.