Then you must be confusing portrait and landscape. Most of the stress from holding a tablet is from the downward force applied to your fingers. If you hold a tablet as any normal person does, one edge of the tablet is anchored between your thumb and index finger and the rest of your fingers support the tablet with an upward torque. The amount of torque can be calculated based on the device's moment of inertia, which is directly proportional to mass and exponentially proportional to distance. As a 16:9 tablet is narrower than, say, a 4:3 iPad in portrait mode, the torque against your fingers is actually less with a 16:9 tablet. If you are experiencing a certain "top-heaviness", it just means you don't have it centered in your hand. This is something you can workaround by positioning it in your hand better or by fanning out your fingers more, but you can't work around the physics of the additional torque due to the higher moment of inertia of a 4:3 device.
Maybe Microsoft should include that in the tutorial they show for the mouse magic corners at first sign-in.