Why does a 10-gigapixel image require 30 gigabytes of hard drive space??

Found in this article:

Draw the Curtains: Gigapixel Cameras Create Highly Revealing Snapshots [Slide Show]: Scientific American:
http://www.scientificamerican.com/article.cfm?id=gigapixel-camera-revealed

..is a reference to this web page from a company that makes the cameras that says it takes 30-40 gigabytes to store one 10-gigabyte image:

360world - Gigapixel images:
http://360world.eu/en/services/gigapixel-images.html

Snipped paragraph:
An image of 10 gigapixels takes approximately 30-40 gigabytes of hard drive space in PSB format. The vistas displayed on the internet consist of 20-30.000 converted JPG images. They are arranged in 5-10 individual layers, so the user's computer will only download and display those relevant to the desired level of magnification. This technology allows gigapixel vistas to display properly, even on average-spec systems.

10 giga by definition, okay, good. Add some extra control data to handle the parsing into the "5-10 individual layers", so as to only serve up the precision appropriate to the client side computer, okay add some more control storage. But tripling it or quadrupling it?

--aec