You can see the problem on this screenshot: http://mybeautifulfrance.fr/wp-content/uploads/2012/12/fb_button_pb.png
It appears when I am logged on my http://www.facebook.com/MyBeautifulFrance page and FB asks to switch to my personal account before loggin.
I understand it is the normal behavior of FB but I don't like the very large white space. Watching at the code, it seems that it is due to the fact that the widh and height of the iframe is defined at 1000x1000px. Is there a way to change this size?