Today, with the introduction of the 3rd generation iPad, Apple has increased the file size that can be downloaded over 3G or 4G networks from 20 MB to 50 MB.
Before if you wanted to download an app that was larger than 20 MB, you had to do so while connected to a Wi-Fi network.
It is logical that Apple did this so its customers can download apps optimised for the new iPad, as these apps are larger in file size because of the increased resolution of the picture resources. This change benefits all users of iOS devices as the screenshot from MacRumors above demonstrates.
