I think it might be an issue with my phone's browser, because it looks as intended if I select "request desktop site"! But doing that means the text won't wrap to fit my screen, so reading comments becomes pretty unwieldy.

I think it's because it treats the "Check spelling" text the same way it treats the text in comments, so when the text on the page is increased by my browser's mobile-friendly mode, so does the "Check spelling" text. I don't know if there's a way to lock to size of that text to always match the size of the text in the buttons? But that's the only fix I can think of.