That's because when you are in the 2nd condition, you're adding the headline in default. After you finish the headline, typing in the comment will have the same font size.
Anyway, we will try to adjust the font size. Thank you for your writing in.
But what if I don’t want to add a headline. For me I rarely use the headline feature.
This is a better idea I think. First font size - should be the default size. Then if user add a comment below - the font size of upper(first) part can increases as like a headline.