>I believe whether or not a response is compressed for IIS depends on the following:
>1) The mime type of the data being returned. This can be configured in IIS to allow/remove more types.
>2) The accept-encoding header of the requestor. For the response to be compressed the accept-encoding header usually will contain gzip, deflate, or both.
>3) The number of times the resource is requested. By default a resource needs to be request at a certain frequency in IIS before it will be compressed. See http://stackoverflow.com/questions/2203798/in-iis7-gzipped-files-do-not-stay-that-way.
>
>To verify that the response is compressed, you need to look at the content-encoding header of the response. This is viewable from the network tab of the developer tools for most browsers. Compressed responses should contain gzip or deflate for that header value.
>
>See http://www.iis.net/configreference/system.webserver/httpcompression for more information.

Thanks Rob, I shall investigate this further as suggested.