ifixit took the new iPhone 12 Pro Max apart and looked at the individual parts. The main difference to the smaller models is the battery and the camera.
The camera has kept its L-Forum, as it was the case with the iPhone 11 models. This design is a bit more expensive, but offers more capacity, because free space can be used more flexibly. With the iPhone 12 and 12 mini Apple is currently using a simple rectangular battery to save costs, and the 5G technology introduced in the iPhone 12 and 12 mini also means that there is less space in these models. The iPhone 12 Pro Max still uses a large battery, even though it is smaller than the previous model (14.13 Wh vs 15.04 Wh).
In addition, the camera module is much larger than the other iPhone 12 models: the Max Pro achieves a much better light sensitivity in dark environments. And this can only be achieved with larger sensors. The housing for the cameras in the iPhone 12 Pro Max is then also almost half the size of the other devices.
You can find even more pictures as always on ifixit.