When I am shooting birds and wildlife, I want to control the iso, shutter speed and/or aperture without taking my eyes off the viewfinder. Further, there are several times where I would tune the focus which means switching to manual and back to auto as needed. All of these need appropriately placed wheels and switches. Small form factor makes everything screen based and hard to access.
The second and probably the more important aspect is heat dissipation. Sony A1 shoots a 50MP image at 8k and can shoot upto 30 frames per sec with compressed raw. Its autofocus system runs at 120 times/sec. All of this processing needs a lot of compute and that heat needs to be moved out somewhere. A large form makes it easier to have decent airflow between components.