It's hard to move fast and have a well engineered product.
Amazon differential is not the quality of their app. Unless they create something really impressive some day, they can only lose customers by moving faster there.
Besides, none of those are small startups anymore. All of them have something to lose.
For my only real experience with amazon (audible) the quality of their app is a negative, I'd much prefer they made an API available. The same can be said of others, I'd much rather native apps connecting to a netflix API than use their awful web player.
So size being a problem seems only true for engineers. A normal user don't care as long as their device still works. And apple won't care since they can use this as an opportunity to up sell larger devices.