With the recent release of the new iPad, app resources (images, videos, etc.) will be significantly larger. Apple has raised the 3G app download to 50mb to compensate for the larger images, but it may still be a tight squeeze. This is especially true for universal apps that have to support low and high resolutions of iPhones, iPods, and now the iPads. Often during the construction of an app we will go through many revisions to the UI, moving new assets in and out of the app. In this confusion, there’s a chance that the developer might forget to remove the image as a resource from the project when it is no longer used.
read more- http://tech.xtremelabs.com/how-to-shrink-your-app/