I find the skyscraper comparison actually quite apt. You build an app that is 10 features, while the main facebook app itself is 100s of features. When you build a building that is more than 20 stories high, you have to change everything about that building. No more wood construction, you have to use steel, you have to think about the bedrock, you need to put in elevators and so on.

I bet if facebook was able to ship an app that only came with the mostly used %20 and was able to download the 100kb of binary for each of the mini features inside their app on demand, it would be a far more scalable application. But they cant, because iOS stops them.