Apple doesn't list iPhone battery capacities anywhere in the tech specs sheet for the iPhone -- presumably because they're so much smaller in real terms than many of the top-end Android smartphones -- but that doesn't mean this information won't get out eventually.
Normally, we'd wait for a teardown to get battery information, but eagle-eyed Dutch site iPhone7.nl spotted the information on the website of the Chinese telecommunications equipment certification center TENAA. The source makes this information about as official as it can be.
According to the information, the iPhone 7 has a 1,960mAh battery, while the iPhone 7 Plus has a 2,900mAh battery pack. This is an increase from 1,715mAh for the iPhone 6S and 2,750mAh for the iPhone 6s Plus -- and makes it a bigger jump from the iPhone 6s to the iPhone 7 than from the iPhone 6s Plus to the iPhone 7 Plus.
This explains the two additional hours of battery life for the iPhone 7 compared to the iPhone 6s, while the iPhone 7 Plus only got an additional hour of battery life compared to the iPhone 6s Plus.