Earlier quoted context omitted.
Apple claims a 10 hour battery life on the 2016, and 9 hours on the 2013 for both the wireless web and iTunes playback tests. Judging by the Comsumer Reports tests though, they would get a shift of 6+ hours from test to test on the 2016 model.
Apple appears to be banking on software efficiency gains offsetting the reduction in battery capacity. They've done this with various iPhone and iPad releases several times. But the trouble with trying the same thing for the Mac is the wider variety of software people run on macOS. Third-party software may not have got any more efficient, meaning the battery doesn't last as long. But if you stick to Mail.app and Safa…
"For the battery test, we download a series of 10 web pages sequentially, starting with the battery fully charged, and ending when the laptop shuts down. The web pages are stored on a server in our lab, and transmitted over a WiFi network set up specifically for this purpose. We conduct our battery tests using the computer’s default browser—Safari, in the case of the MacBook Pro laptops."
CR goes into great detail about their methodology and reasoning for their results.