As said, they are different products.
Opera for Android uses Chromium and traffic doesn't go through Opera's servers unless you are using data saving mode. Also, (almost) all work is done in the client side.
In Mini, all traffic goes through Opera's servers and data is more or less compressed depending on the mode you choose, high or extreme. In high mode Mini uses Android's webview and in extreme mode it works like old Mini, with hthe user getting a snapshot of the page after it have been processed in Opera's servers.