Underline width doesn't scale with font size

With certain fonts, the underline is only 1 pixel wide, even when the font scales up very large. Most fonts the underline scales properly, but with a couple fonts it doesn't. I'm attaching a font (Gentium, a well respected academic font) which demonstrates this problem. Simply open the textedit demo select some text and add underlining at a large font size and you'll see the underline is only 1 pixel wide. This isn't a regression, and I'm pretty sure has been like this since 4.0. Thanks!