Confused about max upload size

I'm getting errors like:

big-email-cover.jpg exceeds the maximum upload size for this site.

Now, the site thinks that's 1MB, so it's clearly not a PHP.ini issue. My PHP.ini allows up to 32MB uploads. Any idea where this is controlled?

(Oh, and bizarrely, when I switched to the browser uploader, it'd take 2MB files. Go figure)