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
To dynamicly resize colorbox you want to say.
colorbox.resize({width:"300px" , height:"300px"})
If you want to resize a color box that loads an Iframe you would add something like this to the head of your target document.
$(document).ready(function(){
var x = $('mydiv').width();
var y = $('mydiv').height();
parent.$.colorbox.resize({width:x, height:y});
});