Why is Samsung not even bothering to change battery sizes?

By all accounts, the S25 series will have identical battery sizes to the S24 series, which were just marginally bigger than the S23 series. They're just making the phones slimmer (big deal).

Meanwhile, the OnePlus 13 just launched with a 6,000 mAh battery and will most likely be cheaper than all but the base model S25.

Why is Samsung focusing on thickness rather than just keeping the dimensions the same and using bigger batteries?