Your comments vis a vis app size are not correct. App size (and battery consumption) have a strong, linear inverse correlation with initial downloads and engagement. There are a number of reasons users may not like large app sizes including being on a strictly data capped phone plan or having bad reception. Even reduction of megabytes showed big changes to engagement (often bigger than feature changes). Reducing app size was and I imagine continues to be a high level strategic priority at Facebook.

I agree with your overall conclusion however. Going to market with something is more important than optimizing for size.

Source: former FB employee