I've been a back-end dev at one of those places. When the front-end is comprised of Android, iOS, Web, and even Windows Phone, a senior and productive back-end

I've been a back-end dev at one of those places. When the front-end is comprised of Android, iOS, Web, and even Windows Phone, a senior and productive back-end team can easily serve multiple front-end teams. In my case, a back-end team of 4 owned everything "non-app" while about 10 engineers owned the various front-ends.