It's really amusing to me when "engineers" start talking about the "scale" of the UI. It's a client. Thin vs. Fat aside, if it's that fat it's almost certainly a bloated mess of redundancy and what is called "overengineering" (which is actually underengineering--that is, a deficiency of the application of engineering and architecture principles to the design of the application).
I had a friend who quit working for Facebook about a month before that slideshow was published, and it was hilarious to see her mock it so mercilessly. She estimated a good 40% was completely dead code, and another 20% was partially dead code (e.g. code for notifying people of "pokes" without any mechanism to view them). She also had some choice words for the engineer who published the slides, but those can mainly be pieced together from the general mockery on Reddit and hacker news.
