Ah. I found why mine were thicker than xrg’s and rawalanche’s. There’s actually a Line Width setting right under the Display Scale setting. I had it on thick, because the UI looked better to me like that. I assumed the lines in the UI were all that this affected. Apparently it affects wireframe thickness too.
There’s really no reason these two unrelated settings should be connected.
That said, after setting the lines to “thin”, they’re still way too thick in edge mode for my taste. I’d really like them slightly thinner than in vertex mode, which looks slightly chubby.