The Galaxy sports a 3.85 V, 9.82 Wh battery, which Samsung claims will provide up to 23 hours of 3G talk time, 12 hours of Wi-Fi internet use, and up to 57 hours of music playback.
With the transition to heavier case materials, perhaps Samsung was interested in cutting weight by trimming batteries. The 9.82 Wh battery in the S6 and 10.01 Wh battery in the S6 Edge feel pretty small compared to the Galaxy S5's 10.78 Wh pack.
The battery is quite a bit bigger than the 6.91 Wh we saw in the iPhone 6, but arguably provides less talk time. It seems like that extra screen real estate takes a lot of juice.