Actually the active area of the CMOS sensor is really quite small. Like ~0.1cm^2. It is the main reason we need so darn many phones (and, as you correctly point

Actually the active area of the CMOS sensor is really quite small. Like ~0.1cm^2. It is the main reason we need so darn many phones (and, as you correctly point out, an appropriate density of phones) in order to have sensitivity. The reason it's still interesting is that at the very very highest UHECR energies, like above 10^21 eV, the density of ionizing particles is so tremendous that it's almost impossible for a (tiny) camera not to see it, if it's close to the core. This is great because unlike the Pierre Auger observatory (which is already about 100% effective at detecting showers above 10^18 eV), we become better and better at detecting events the higher the energy. So while P-A has pretty much constrained the UHE spectrum up to about 10^20 eV, the limitations on its size (as in, how much area it covers on the ground) means it can't do any better, since the higher-energy events become so rare that the probability of them happening over their patch in the desert is quite small. The hope with CRAYFIS is that, given global coverage, we can be the most sensitive measurement of these ultra-rare events at energies above what P-A has been able to observe.