It doesn't have a width assigned to it, so it's taking up all of the available space. Since your image is being floated to the left, it is taken out of the flow of the document, so your comment box has nothing to butt up against - hence it being full width.

Just adding 'float: right' to the comments-wrap like so will fix it:

#content #comments-wrap {
	 padding-top: 10px;
	 float: right;
}

This is located on line 485 of your style.css