If you think a five pound laptop is considered "heavy" be thankful you weren't trying to do this 6 years ago when $3k bought you an 8lb computer with a 1Ghz processor and maybe 256mb of ram. It's amazing the mbp is as light as it is and the macbook is right in line with comparable offerings. Engineering means that the smaller machine is going to be lower spec. 32gb SSD on a computer whose OS takes up 21GB? Keep dreaming. It would be slower than your macbook.
If you look at the ultraportable computers out there most of them are 1.5 or 1.6GHz single core with integrated graphics, because the casing can't take the heat from anything else. It wouldn't run 3d better than your macbook, that's for sure.
actually lenovo and sony have ultraportables with discrete graphics, 12/13" monitor and lighter weight than any apple laptop