https://wccftech.com/galaxy-s9-stacked-logic-board-bigger-battery/

The Galaxy S8 comes shipped with a 3,000mAh battery, with the Note 8 featuring a 3,300mAh cell. Despite the increased display size, the overall footprint of both phones has been greatly reduced, leading to Samsung making a few compromises on the battery side.

In 2018, the company might not be forced to make such tradeoffs because the Galaxy S9 and Galaxy S9+ have both been rumored to come with a changed motherboard design. This design will bring several perks to the table, as you will soon find out.