The same real world react app went from 40MB to 20MB.
They don't ship minified javascript to final app, but bytecode, which they say is 1/3 smaller (both minified). They have moved the parse and compile stages to ahead of time, making the VM smaller, and giving the bytecode benefit. The bytecode benefit also means a quicker time to live/first reponse from 4s to 2s in their example.
Link to the talk, from FB employee announcement made at React Conference today, 15 minutes length. Linked 2 mins in, because the audio is poor to start.