Well we shouldn't need huge batteries if Android was fixed. Have you seen the batteries Apple put inside iPhones? iOS is great for their hardware margins.
Note 4 is a 5.7" screen. There's plenty of room for Samsung to ship a high capacity battery. The Blu Studio Energy is only a 5" but has 5000mAh. If reviewers started giving out bad reviews to anything that can't last a 24h day, other OEMs would fall in line.