The strange thing about jetpack's setup is that it makes the gallery width the content width coded into the theme and it's not customizable (which doesn't make sense). The theme has a content width set in the functions.php file like this:
if ( ! isset( $content_width ) )
$content_width = 793;
I may need to change that to be 1140 which is the full width of a page in this theme. But this might give you some more info, plus talks about a plugin you can try out:
The other option you have is to not use the jetpack gallery version.