Well, all mobile processing units (CPUs and GPUs) are horribly weak next to their strong and dedicated desktop counterparts. Just for a weak, general comparison:
According to NVIDIAs own GeForce website the 660M scores a little above 10,000 on 3D Mark Vantage while the GTX 660 scores almost a 30,000. So even just off a benchmark you can see the non-mobile counterpart (according to the name of the cards) is almost 3x more powerful. The GTX Titan and GTX 780 both score around 50,000. The GTX 650 even scores about 15,000.