They have a much higher-touch enterprise sales process, so my guess is that most of the extra employees vs. DropBox are sales, customer service, relationship management, etc. Even after the sell, big clients tend to want dedicated account managers, dedicated support staff they know by name, etc. (Still, it does seem like a lot.)
Are you fully aware of what they offer? They have all kinds of well-known and arcane certifications, integrations with a few dozen products that are only relevant to large enterprises, a pretty big third-party ecosystem and the API to support them, and the large team required to sell and support these tools.
It's not just file storage for five bucks a month.