The "problem" is here in your style.css - line 58
#content p {
margin-bottom:9px;
width:400px;
}
The width:400px property is conflicting with the default width set by my template.
If you remove it, it will properly work.
I say the "problem", because I think this might be a customization that you added to your style.css... So it might be something required for your other pages. In that case, like I said before, you should adjust the template (not for the margins like I first taught) so that it uses your 400px width instead of the 300px width I set as a default.
The way I've built the plugin, it requires a set width in order to work properly. Maybe there are other ways to make this a bit more flexible, but it would require more code, which will make it even harder for people to look at the source and figure things out I think.
Please let me know how this goes and if you can manage to adjust this to your liking.