I think the limitation is volume. Scale a phone by 10% in all directions, and its volume goes up by 33% (-ish). Its display grows by only 21% in area, though.
So, roughly, you get 10% more power per pixel or square inch of screen.
And I think it’s even worse for the smaller phone. The faster CPU and GPU and more memory you need to drive 21% more pixels and to light 21% more screen area do not take up 33% more volume, and Bluetooth and cellular radios stay the same size, so you can scale the battery volume by over 33%, and, likely, its capacity even more, as the fraction of the battery’s volume that is lost to the battery housing can be lower for larger batteries.
Scale a phone by 30% in all dimensions, and the effect is a lot larger; 69% more screen area, but 120% more volume, so almost double the power/pixel.