I'd be curious to hear too!
I knew somebody at Twitter years ago, and he said their mobile teams were bloated and individuals didn't do much. I mean, the Twitter app is quite simple overall, how can you have hundreds working on the UI?
My guess is it becomes hard to contribute quickly at an individual level due to process, lots of overhead with planning, and perhaps codebase not being clean, or just overly large that you don't have good sense of the scope.
Would love to hear from somebody that's worked on a team like this