Ahoy captain,
I had the same problem. I solved this by tweaking CSS line 577. By default, the two column layout (which I am assuming you are using) is set for a right column of 50%, which is obviously too big for a sidebar. I used the following custom CSS in the theme options layout area to make it smaller:
.twocol {
width:30%
}
Use whatever percentage you wish, but I found that 30% looks good.
Hope that helps!