Yes but Microsoft has Windows, Office, Xbox, Surface, Bing, Skype, etc. PayPal is a big business with many features, but 13,000 people is a lot for just that one thing. I can't believe even half of them are developers.
When you have a quarter billion users of a money transfer product with 24/7 support, you probably need a pretty big service staff. I'd imagine customer support, tech support and administrative staff outnumber developers.
You will be surprised how employees work on one thing called iPhone.
A large product like PayPal might have dozens of sub systems. Its difficult to imagine from the outside the dynamics of large companies and how they function.