The Kindle files are twice the size because PG chooses to implement them that way.
That's what kindlegen outputs. I don't touch kindlegen's output in any way.
You are choosing to run kindlegen, and distribute mobi files, without compression, but the epub files are being distributed with compression. You can't pretend to tell me you haven't even read the kindlegen documentation.
It is trivial to get free files on the Kindle Fire, as I have already explained to you, by using the Amazon "Send to Kindle" applet on your computer.
That's ridiculous. What would you say about a car you cannot drive to the filling station but instead requires you to buy a pickup truck to bring some gas cans back to the car? Trivial eh?
I would say that if you actually tried "Send to Kindle" you would find it is in fact "trivial" to use, and an attractive approach to getting free books on your ebook reader, and no more requires driving to the gas station than filling up my car, which is electric, so that both my Kindle and my car are "trivial" to load up at home, and neither requires your [confused, hypothetical] trip to the filling station. I think you must be confused with Apple devices, which always seem to require a trip through the Apple store.
Obviously you still say things without doing any research.
https://play.google.com/store/devices/details?id=nexus_7_16gb
I tried this, then, and I try it again now, and it says that this option doesn't exist. So who is saying what? Further, I have tried to see the Nexus 7 in a half dozen stores which carry tons of ebook readers, I ask to see the Nexus 7, and they don't have any. Which might explain the ebook survey numbers -- look under "Nexus 7."