Almost all of the things in your list are harder to do at scale, especially anti-spam/anti-fraud. Plus, all of this needs to be done in compliance with the legal regimes of many countries where Twitter operates -- especially regarding advertising and privacy. Having seen how some other large web services work from the inside, I'm not surprised Twitter needs a lot of people.
It would be possible to pull this off with far fewer engineers, but not sustainably. People aren't robots, they can't work 80-hour weeks forever. People also need to be able to take vacations, and sometimes they will leave the company -- so it's not a good idea to run super lean. Plus, when companies run lean, the security and privacy aspects of the engineering tend not to be done very well (if at all), and for a social network that is a serious issue.
Twitter was definitely bloated, but I've seen and heard of worse bloat at other tech companies.
It would be possible to pull this off with far fewer engineers, but not sustainably. People aren't robots, they can't work 80-hour weeks forever. People also need to be able to take vacations, and sometimes they will leave the company -- so it's not a good idea to run super lean. Plus, when companies run lean, the security and privacy aspects of the engineering tend not to be done very well (if at all), and for a social network that is a serious issue.
Twitter was definitely bloated, but I've seen and heard of worse bloat at other tech companies.