Dual GTX 750 needs a good heatsink(s) to dissipate heat. That usually means a lot of copper. And copper is not light. Even if it was an aluminum heatsink it will not be light. However, for the same volume of metal copper conducts heat more than 2x better than aluminum can.
To the best of my knowledge silver is the best conductor of heat; but it is only about 9% better than copper... and much more expensive.
MacbookPro .. that Asus that was linked.. its a slim market out there for a really powerful machine that is leightweight and portable. The Razer blade (which is marketed as a gaming rig). Couple others out there, but your not going to pick them up for less than 2,000