Why is "font-size: 1em" bigger in IE10 than Chrome?
I've "solved" my own problem, 'cause I'm a fool.
IE10 has two settings, both under the "View" menu, which change the size of what's displayed. I'd made sure that the "Zoom" setting was set to 100%, but completely overlooked the "Text size" setting, which was set to "Largest". Setting it to "Medium" fixed my issue, and made it so both IE and Chrome displayed identically.
The size of em in pixels is related to the font type & size you're using, the resolution of your screen (depending on OS, browser), and possible further OS and browser settings - eg "Show fonts +10%" may alter the em value.
According to Rudu in this post.