The problem is that if you don't play by their "standard box" rules, you may not end up in the truck at all. The users browsing the AMP results may never feel

The problem is that if you don't play by their "standard box" rules, you may not end up in the truck at all. The users browsing the AMP results may never feel compelled to scroll down to see non AMP results. So a publisher's choice isn't: "standard size for faster delivery", it's: "use AMP or receive less traffic than you're currently receiving"