Share to Flickr - won't send "Actual Size"

I've shared a few photo sets to Flickr from iPhoto 11. This works well, except that the "Actual Size" setting seems to have no effect. Every photo I upload shows an original size of 1024x768 instead of 3648x2736. This occurs if I upload to an existing Flickr set or create a new set.

Oddly, if I just share my photos to the Flickr photostream, the full-size images are uploaded.

Am I doing something wrong here? I do have a Flickr Pro account, so that's not the issue.