The AJAX content loaded in a Colorbox has some JavaScript included that resizes things within the content. Colorbox determines its sizing based on the sizes before all of th
Well, I haven't used Colorbox before but I have used similar things, and if I understand correctly, what you need it to do is change the style of the box after something's been loaded.
If you find what id/class values are applied to the Colorbox, you could make it so that when the AJAX content is loaded, a function is called to change the style of the appropriate part of the Colorbox.