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.
Note sure how I would be confusing portrait and landscape. Regardless, if you hold the Surface in portrait orientation it becomes very heavy to hold. I hold it as you mentioned, but most tablets are gripped in the lower corners. You are saying that I should grip it more in the middle. I would assume you don't have a Surface yourself because toward the middle is where the capacitive Start button is. Get anywhere near it and you'll be flipping back into the Start screen. Thus you are forced to keep your palms in the lower corners. So yes, I am holding it right and yes, it's still heavy.
Surface is mostly a two-handed device, especially if you hold it landscape. This is one of the reasons I rarely reach for the Surface as tablet of choice.