Procedural content - that's one of the big technological innovations of Spore. Basically, instead of storing the model for the creature, Spore stores the instructions for how to build the model. Here's a really simplified example. Let's say that for some reason, I wanted to send you a sine wave. Don't ask why, I just do. Now, I could calculate the date for every point on that sine wave, but that would be a helluva lot of data to send. So instead, I simply send you the formula y = sin(x), which you can use to generate the exact same sine wave. Of course, in Spore, the creatures are much more complex than sine waves, and so the mathematical formulas are, too, but that's the basic principle.