It seems RadScriptManager tries to compress the combined output (compress=1 in the url) but for some reason it is not successful (may the ISA Server strip the Accept Encoding http headers?).
Also, you are using an older version which probably suffers from this problem
Here is a little tool
that will help you with this task.
In the attached web site you'll find a ScriptResourceDetector.aspx page. Place all of the controls that you use on your site in the panel on the page. Upon loading this page you'll see a list of the scripts used on the page that you should directly paste in the Scripts section of the RadScriptManager.
Ideally, RadScriptManager will be placed in a master page, so you don't have to replicate this list across all pages. It'll then serve the same script on all of them, as long as the pages use a subset of the controls that you've used to build the list of scripts.
This way that big script will be loaded only once and then its cached version will be used.
the Telerik team