The maximum size allowable for an Android app on the Android Market is 50MB. 
The maximum size allowable for an iPhone app on the App Store is approximately 2GB. 
To answer your question, when you download the Android version of this game, you are downloading an application that will download all of the data files either to your phones internal memory or SD card. Due to the nature of the game, this is likely to account for the majority of the size difference unless different compression methods are used for resources such as the sound, textures and 3D models.
The iPhone's App Store has a much bigger file size limit which in this case means that all of the resources can be bundled into the initial download. The phone will not have to fetch any additional resources.