High Dynamic Range Photography-
A set of techniques that allows a greater dynamic range of luminances between light and dark areas of a scene than normal digital imaging techniques. The intention of HDR is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows. HDR images require a higher number of bits per color channel than traditional images, both because of the linear encoding and because they need to represent values from 1 millionth to 100 million bits or more.

Average sizes:
HDR - 80Mb*
PNG - 30Mb
JPG - 5Mb