> Why? If your app is used by billions of people, surely you can afford a few additional testers and engineers?

Unfortunately, in the real world, # of bugs solved per unit time does not scale linearly with # of developers - and, you eventually reach enough people that you can't effectively coordinate changes without wasting more time on processes than you're gaining by adding another person.

I've never worked at Apple and I don't know anyone on the Photos team, but I imagine a company at that scale probably has a good idea as to the optimal number of developers working on one application. Optimal to Apple probably involves optimal money spent:money earned ratio, not most bugs fixed per unit time, but I would wager those numbers are pretty similar anyways.