If you submit images to picture libraries they often need to be the size you mention, because they are often blown up to a larger size - they could be used for advert boards etc. They therefore need to be high res and large size.

If files are 16bit that nearly doubles the size over 8bit. Layers in a file will greatly increase the size. PSD files always seem to take up more space than TIFFs. Files from a Canon Mk1d, or something like it will be larger, due to the high pixel rate of the camera. It's easy to make a file bigger, just change the size and user interpolation.

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