Android apps try to fill the screen of the device they're installed on by default. However, I was having problems getting an app to expand to fill the whole screen, despite making sure the layout was set to 'fill_parent' and screen size was 'anyDensity'.
After a bit of searching, I found that apps only expand to fit a screen if you declare a minimum SDK target in the manifest:
<uses-sdk android:minSdkVersion="4" />
This line of code will mean that Android will automatically handle larger screens (which, by default, means it will try to stretch the app to fit).