The batteries powering the Samsung Galaxy S9/S9+ might not be bigger, but they should perform better
Galaxy S9+, respectively. After all, these are the exact same capacities found on the cells powering the Galaxy S8 and Galaxy S8+.
Even though Samsung doesn't apparently plan on raising the capacity of the battery in each phone, they have been changed in some way or else ANATEL would not have felt the need to certify them. Additionally, the Snapdragon 845 mobile platform under the hood of both devices (in the U.S.) is designed to help phones achieve all day battery life. And with Quick Charge 4+, a cell can be 50% filled with energy in just 15 short minutes. In other markets, Samsung's own Exynos 9810 SoC will be powering the handsets, and is expected to bring as much as a 15% savings in energy consumption over last year's chipset.
Therefore, even if Samsung keeps the capacity of the two batteries inside the Galaxy S9 and Galaxy S9+ the same as last year's models, there should be improvement to the battery life achieved by both phones. Those who want a longer lasting power source might want to wait for the Samsung Galaxy S9 Active. This year's rugged variant of the Galaxy S8 happens to carry a 4000mAh battery.
source: SlashLeaks (1), (2)