I haven't looked at what Google does recently but you're likely getting most of Chrome in a separate package (e.g. the system WebView). Chrome and Firefox are of compareble size IIRC (56MB vs 65MB)
We're working really hard to make extensions a first class product on Firefox Preview. The team rewrote the Android side of things from scratch so there's a lot to rebuild but we're getting close. Stay tuned!
The plan for Servo is to integrate it in Gecko piece by piece. In this sense GeckoView (which uses Gecko) will use Servo eventually. It already has WebRender and Stylo from Servo (like the rest of Gecko-powered products).
> Keep in mind that GeckoView may not be used to build a browser. Rather it's often an engine used to drive an application built with web technologies. So a developers requirements for updates are different than an interactive browser of arbitrary web content.
It most certainly can :) the new Firefox for Android uses GeckoView.