Key #2: Your App Must be 20MB or Smaller
Blockbuster apps get many of their downloads directly from users' iPhones or iPads. A game is very much an impulse buy. If someone tells friends about a great app, they almost immediately go to their mobile device to search for the app and download it.
There is a 20MB limit for downloading apps directly from an iPhone or iPad. Any app that is over this size limit must be downloaded from the App Store from a Mac or PC.
If your app exceeds this limit, and users want to download it immediately from their mobile device, they won't be able to until they connect to their Mac or PC. The chances of them remembering to download your app diminish considerably with this extra step.
Now, please note that some apps just can't stay under 20MB due to huge graphic files and so on. This doesn't mean your app can't be successful. I'm just pointing out that the top 10 game apps are usually under 20MB. Angry Birds is around 1112MB, for example.