I see a lot of strange assumptions here. The main strange assumption appears to be that a larger sensor will help against shake. But that is only relevant if the displacement of the sensor is a significant factor for motion blur. This may be the case for macro photography or closeup photography. However, the camera is not held at its center of gravity and both hands will have significantly uncorrelated shake. That means that the resulting rotations will become much more relevant once the object distance is not dwarved by the focal length.
Now with a bigger camera you have several effects: overall mass will be larger, meaning that equal forces lead to smaller displacements. Overall size will be larger, meaning that equal displacement difference between the hands will lead to smaller angles of rotation. Putting this together, rotational inertia will be larger. Rotational inertia, the ratio between angular force and resulting angular momentum change, grows with the square of the distance to the rotational centre and the weight (of course leverage also grows with the distance, but only linearly).
Now there also is motion compensation with modern cameras, using lens movements and/or sensor movements. When smaller angles need to get compensated, this kind of compensation has more room to go before it has to give up.