I've only noticed this since I hit 2000 rep and gained the editing privilege, but I'm not sure the two are necessarily related. It seems that after I edit a post containing equations, the equations are rendered bigger than usual. Here's a screenshot:
You can see my answer above, which is normal, and then the answer below that I've just edited (and you can see my downvote, gasp! anonymity ruined!). In the edited answer, the $x$ is a little larger than similar $x$ in my answer above. When I re-open that page, the effect is gone - it seems to be only there immediately after editing.