I'm really wondering here. If you want people to actually like the game your forcing them to watch an advert on, is it really in your best interest to make the x as small as you possibly can? and likewise, do you need to force us to click said x multiple times? do you honestly think that this is how you get people to install and try games, or are the advert companies just using this to lie about the clickthrough rates?
Your basic theory of "app developer wants me to like the app" is flawed. They don't want you to like the app, just to watch as much ads as possible. And since they get more money if you click the ad, the X has to be extra small. But they cannot remove the X since guidelines say they have to provide a way to close the ad without interacting with it.