even if the game is cached forever, with every small update it needs to download the whole game again, am i right?
No, it will only download the changed files and save them to disk. It will not download any files that have not changed. So you can even have a 100mb game, and updating a single 10kb file every few days will only download the new .appcache file (a few kb) and the single changed file. Browsers aren't that dumb, they won't waste all your bandwidth :)
In other words I believe it automatically works exactly how you want already, and you do not need to do anything special at all.