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.
Then flip the Windows button around to the other side.
Thus you are forced to keep your palms in the lower corners.
No, you're not. See above.
So yes, I am holding it right and yes, it's still heavy.
If you're holding it in the corner, the aspect ratio isn't going to make much of a difference. It's going to be heavy no matter what. So you are holding it wrong.
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.
In my use cases, my hands aren't needed to support the majority of the weight, whether it's with one hand or with two hands. I either have it on a table or resting on my belly if I'm lounging on the sofa.