They probably have proportionately far fewer security employees, just far more effective ones.

All snark aside, I'd guess that the vast amount of Uber security staff's role is investigating driver fraud. Uber grew so immensely fast because they offered compelling sign up inducements to drivers, and in lots of cases those were easy to game. It's why Uber got caught "fingerprinting" iPhones by Apple so they could detect whether the same phone was being used to sign up for multiple driver accounts to double dip the signup bonus programs.