Maybe the lack of human resources is offset by the lack of managerial bottleneck (you can just get stuff done and solve problems immediately with your keyboard without the need for weekly meetings or approval from some non-caring boss)? I've always been astounded too.
The end of the book 'The PayPal Wars' gives fascinating context on how development just completely dies in a corporate environment. They went from being able to roll major new features in a weekend to requiring months just to schedule a meeting to discuss a font change. It's a fascinating read when we now look back at how PayPal just stagnated under Ebay's ownership.
I think twitter is a few thousand total employees, a fraction of that is engineers.
And a good chunk of it is maintaining & improving a large throughput backend system that is one of the top 10 in the world in terms of size and data volume.
And on top of that, an even smaller fraction is the people who think of and implement new front end things that you would interact with and think of the app changing. Like a lot of companies, it's probably a 10:1 ratio as far as backend:frontend people there are at the company.
As to why twitter changes so little, the company itself doesn't really understand why it's successful, so there is a lot of conservatism inside of it to not screw with the golden goose.
