That’s odd. Under the Tablo Settings, I use the “720p Roku / Chromecast” recording quality and the video size is not smaller on any of my playback devices - Roku 3, Fire TV, Fire TV Stick, iPhone, iPad.
Where did you change the Chromecast settings exactly? From “720 to 720p high bit rate” as you said? Are you casting to the Chromecast from your Android phone using the my.tablotv.com website via the Chrome browser rather than using the Tablo Web App?
The next firmware update will include support for drives larger than 2 TB, but no multiple drives. In the meantime, I have found the most user friendly tool to save recordings off your Tablo is Tablo Exporter by Jestep, see the link below: