Amazon could have switched to epub instead of botching KF8, but chose to be deliberatley incompatible to the rest of us.
The DRM epub world is incompatible with itself, so presumably Amazon saw this issue as a "no difference from everyone else." It is strange that you self-identify as being "epub" rather than "PG."
It could also be that Kindle files are twice the size as epubs, or that is is practically impossible to get free files on the Kindle Fire.
The Kindle files are twice the size because PG chooses to implement them that way. 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. While you may find some theoretical advantages to the Google Nexus 7, in practice Google isn't selling it anymore, so those advantages must remain purely theoretical.