It's not really the physical size of the antenna that matters, it's how much gain the antenna has. If I remember my antenna stuff correctly, every 3dB approximately doubles the signal from the antenna, so the 12dB gain that the Pringles can antenna has is equal to about a 16x increase in signal.
[Edit] After reading those sites, I'm partially wrong.... it looks like the bigger the antenna (in certain cases), the more gain it has.... [/Edit]
JW