When I started at Google, everybody said the same thing. What do you need 3000 people for? They thought it should require like 50 people. It's just a box on a web page that you type into. But there was a lot going on behind the scenes to make that happen :)

I don't know anything about Asana, but there is this iceberg illusion where people judge the size of a company by the size of the UI. The size of the company is probably more related to the size of its customer base, among other things.