It is still an N frame, and the cylinder can't be any bigger. There's only so much room in the hole in the frame. If they made the cylinder bigger, they would have to make the hole in the frame taller to accommodate it, which would require them making a new frame size.
That's what they did when they decided they wanted to put 38 Smith & Wesson in their little pocket pistol. The i frame was a six shot 32. They wanted to make it a 38, but the hole in the frame was not tall enough for a cylinder for 38 - even only putting five 38 in it. So they made a new frame with a slightly taller hole and called it the improved I. Then they wanted to put 38 Special in that gun but the hole in the frame was not long enough, so once again they had to make a new frame with a longer hole, and they called it the j frame.
The only thing they could do to make that six shot 357 an eight shot 357 would be to make the holes closer together.
I carry the 686 3" and it is heavy and painful to shoot. I can't imagine what this will be like being a lightweight frame. If it is anything like the 642 airweight I would be afraid to shoot it without injecting lidocaine in my hand and wearing gloves. It is a beautiful gun though.