https://wccftech.com/2021-12-9-inch-ipad-pro-has-a-larger-battery-compared-to-the-2020-model-11-inch-model-remains-unchanged/

Here is something interesting: the latest 2021 12.9-inch iPad Pro features a larger built-in battery compared to the 2020 model.

We highlighted how the 2021 iPad Pro is thicker and heavier compared to the 2020 model, if you are going for the 12.9-inch option. It makes sense - Apple had to squeeze in a Mini LED display hence compromises were made.