I don’t use this particular plugin for WordPress, but I do use it in non-wordpress implementations. Try changing the #cboxTitle line in the css. I have used the following:
#cboxTitle{position:absolute; bottom:10px; left:25%; text-align:left; width:50%; color:#333333; border: 1px solid #999999; padding: 10px; font-size: .9em; background: #f4f4f4; }
This will extend the caption above the image, not stretch the bottom of the caption area. Doing that is a little more involved as it is an absolutely positioned div, not nested within the area itself. Let me know how this works for you.
