Please note that XCOM: Enemy Unknown for Android does not support all Nexus 7 models. XCOM will run properly on Nexus 7 models released during Summer 2013, but due to the game's memory requirements, it will not function with older models (released Summer 2012).
The development team is constantly reviewing the list of supported devices and working to get our titles on as many as possible, but there are no plans currently to bring XCOM to devices with less than 1.5 GB of RAM.