hmm AIR... doesnt iOS have no flash?
and then on Android not every device has flash either and not everyone has this huge framework instealled. also it probably takes a lot more ressources than a native app.
Even though AIR is flash based technology it is not the same as flash browser plugin. So iOS has no flash browser plugin but AIR works fine for iOS, you can check Adobe's website for more information.
I agree on devices with low space for the applications the AIR installation will be huge if it will have to be installed separately. However the Adobe AIR 3 SDK has a new feature called Captive Runtime (I think it existed in the previous version only for iOS). This feature bundles the AIR runtime with the application, so that the user doesn't have to download-install it separately and I guess it will be smaller than installing the framework separately.
On the resources side I don't know how much is it supposed to use as a native application but during the profiling the current code base was using 6-7MB of memory max during the first test, still it is on alpha version and the code is not that optimized.