https://www.wired.com/2016/03/epic-story-dropboxs-exodus-ama...
That is just a portion of their infrastructure engineering activity. They also have product engineering - frontends, backends, mobile clients, integrations, etc.
It's not just a server under some college kid's desk.
This part really was highlighted to me when I contacted support over a bug. They linked me to an introduction to Dropbox which was kind of offensive since I've been a user for 8 years and they completely ignored the bug report it seems :-(
I guess it hadn't registered in my brain that a company with as many users as they do has customer service not unlike a larger Acme Co. tech support desk.
It is overkill!
Where else would they be?