If a page (printed or on a screen) is designed well, it’s obvious what is a header and what’s body text. Normally, the header is larger, but it doesn’t have to be. There are all kinds of ways you can set your headers apart—make them a different color. Put them in letter spaced all caps. Italicize them. Make them a different typeface. Make them bold—or light.
I’m not saying that using size to set things apart is a bad idea, I’m saying that it’s not the only idea. Try something new!