I don't think the data that's sent from Opera mini to the server is so interesting, rather the data sent from the servers to Opera Mini, i.e. the actual web pages. All I know is that the images are compressed with WebP.

That is true for opera browser most likely. http://pike.lysator.liu.se/about/pike/history.xml I think Per Hedbor put the 'spinning' Pike compiler under run at Opera.