Well, the 12.9" has the same 4:3 screen aspect ratio as the 10.5" iPad Pro, and thus the 12.9" will have large black bars on 16:9 material. The most efficient use of the screen real estate is on the 11". While the image on the 12.9" will be larger, it will be less efficient in using the screen space due to the shape of the screen.
The 12.9" iPad Pro is 2732x2048. For 16:9 material, it is 2732x1536 = 4.20 Megapixels.
The 11" iPad Pro is 2388x1668. For 16:9 material, it is 2388x1343 = 3.21 Megapixels.
The screen size of the 12.9" iPad Pro is about 40% larger than the screen size of the 11" iPad Pro, but the 16:9 image size is about 30% larger.