JetPack Photon incorrectly resizing images

Here’s a comparison screenshot of an image on my site with and without Photon active

I tried following the advice here and added

if ( ! isset( $content_width ) )
    $content_width = 1020;

to functions.php but nothing changed, the image is still 780 pixels (the “full” size it would be when the widget sidebar is on the page.

Is it because it is still using the cached images from before I added that code and it will eventually re-cache? How do I get photon to serve up the correct size when on a page without sidebar widgets?

thanks!