Most consumers look at megapixels first when buying a digital camera, but the sensor size actually is a more important factor. The sensor captures light through small photosites or pixels, and it’s the size of each pixel that determines the amount of light, color, noise, sharpness, and dynamic range. Even if a camera manufacturer is able to cram a high number of pixels onto a tiny sensor, a camera with a larger sensor will produce better images. 

Point-and-shoot cameras have the smallest sensors, although they have improved significantly in recent years. Mirrorless interchangeable-lens cameras are a new generation of compact with considerably larger sensors than a point-and-shoot but without the bulk of a digital SLR. The largest sensors usually come in digital SLRs, which are bigger and heavier than compact cameras but produce the highest quality photos.

Below is a sensor size comparison for the leading digital cameras of 2013:
 

Point-and-Shoots
Apple Iphone 5: 4.54 x 3.42 mm = 15.5 sq. mm
Olympus TG-1 iHS: 6.17 x 4.55 mm = 28 sq. mm
Fujifilm FinePix XP200 = 6.16 x 4.6 mm = 28 sq. mm
Nikon S9200: 6.16 x 4.6 mm = 28 sq. mm
Nikon P330: 7.4 x 5.6 mm = 41 sq. mm
Canon S110: 7.4 x 5.6 mm = 41 sq. mm
Sony RX100: 13.2 x 8.8 mm = 116 sq. mm
Sony RX1: 35.8 x 23.9mm = 855 sq. mm
 

Mirrorless Interchangeable-Lens
Nikon J3: 13.2 x 8.8 mm = 116 sq. mm
Olympus PEN EPL-1: 17.3 x 13 mm = 225 sq. mm
Panasonic Lumix GF6: 17.3 x 13 mm= 225 sq. mm
Olympus OM-D E-M5 = 17.3 x 13 mm = 225 sq. mm
Sony NEX-5N: 23.5 x 15.6 mm = 366 sq. mm
Sony NEX-6: 23.5 x 15.6 mm = 366 sq. mm
Sony NEX-7: 23.5 x 15.6 mm = 366 sq. mm
FujiFilm X100S: 23.6 x 15.8 mm = 372 sq. mm
Leica M9: 36 x 24mm = 862 sq. mm
 

Digital SLRs
Canon EOS Rebel T4i: 22.3 x 14.9 mm = 332 sq. mm
Canon EOS 60D: 22.3 x 14.9 mm= 332 sq. mm
Canon EOS 7D: 22.3 x 14.9 mm = 332 sq. mm
Nikon 3200: 23.5 x 15.6 mm = 366 sq. mm
Nikon 5200: 23.5 x 15.6 mm = 366 sq. mm
Nikon 7100: 23.5 x 15.6 mm = 366 sq. mm
Nikon D800: 35.9 x 24 mm = 861 sq. mm
Canon 5D Mark III: 36 x 24 mm = 862 sq. mm