Programming Android apps in jython

Jython doesn't compile to "pure java", it compiles to java bytecode - ie, to *.class files. To develop for Android, one further compiles java bytecode to Dalvik bytecode. This means that, yes, Jython can let you use Python for developing Android, subject to you getting it to play nice with the Android SDK (I haven't personally tried this, so I don't know how hard it actually is) - you do need to make sure you don't depend on any Java APIs that Android doesn't provide, and might need to have some of the Android API .class files around when you run jython. Aside from these niggles, your core idea should work - Jython does, indeed, let write code in Python that interacts with anything else that runs on the JVM.

As long as it compiles to pure java (with some constraints, as some APIs are not available), but I doubt that python will be of much use in development of android-specific stuff like activities and UI manipulation code.

You also have to take care of application size - that is serious constraint for mobile developement.

Yes and no. With jython you can use java classes to compile for the JVM. But Android use the DVM (Dalvik Virtual Machine) and the compiled code is different. You have to use tools to convert from JVM code to DVM.