Dig! Fight! and Explore in a more
expansive world over 2.5 times the size of what has been available
thus far for Mobile Terrarians!
If your device has less than 1GB of
memory, you may experience performance issues if you create an
Expanded World. The option to create Expanded Worlds is not
available on devices with less than 512MB.
http://forums.terraria.org/index.php?threads/terraria-mobile-update-expanded-worlds-more.41313/