I heard there are something like 400 mobile developers working on the FB apps. I don't know there's any way to keep a team that size from producing bloat with every revision.
That seems like a useful engineering problem for Facebook to try to solve, rather than this bullshit rocket science optimization stuff that adds yet more complexity for only incremental gains.
My take: If you can't write it fast enough in Java, it is probably not a good mobile app candidate in the first place. The Dalvik optimizer and the JIT yield a regularly reasonable experience for me.