What Franco related is true, but the Canon 5DIII as part of 90 pounds of gear is hardly what I would call ultralightweight.
What Franco related about sensor size is true. The larger the sensor, the more capable it is likely to be in gathering light. Also, at least in the Canon realm, the G15 sports a CMOS sensor, not a CCD sensor. CMOS sensors are much quieter than CCD sensors. That is in the sense of color purity and lack of color noise. CCD sensors excel in being cheap since there are so many on the market. Further, the light-gathering photo sites on the sensor are farther apart on a large sensor. That means that they can do their job and not get so much noise from adjacent photo sites.
Lastly, as you get into larger cameras, they can accept larger lenses. By that, I mean lenses full of heavy optics that gather light more easily.