At https://www.lesserwrong.com/sequences, when I open up Chrome’s dev tools on my 13 inch Macbook Pro with a macOS Sierra OS, the computed value of font-size for the paragraph of main text is 20px, and the computed value of line-height is 25px. But on https://www.lesserwrong.com/posts/AmaWMMWPzuQ62Ernf/against-individual-iq-worries I’m getting the same values you mentioned. So I assume it at least depends on what type of post is being displayed.
At https://www.lesserwrong.com/sequences, when I open up Chrome’s dev tools on my 13 inch Macbook Pro with a macOS Sierra OS, the computed value of font-size for the paragraph of main text is 20px, and the computed value of line-height is 25px. But on https://www.lesserwrong.com/posts/AmaWMMWPzuQ62Ernf/against-individual-iq-worries I’m getting the same values you mentioned. So I assume it at least depends on what type of post is being displayed.