There's a width that's being set on the button itself. That's done by Google, and not something you can modify at the time the page is rendered. You can try adding the following style to your stylesheet, which looks like it may help.. but I can't be certain:
#___plusone_0 { width:46px;}
Hope that helps, but if not, let us know when you've implemented that style and we'll test and suggest something else if possible.
Thanks,
Alex