So WhatsApp had did have bigger team (you're comparing 50 engineers vs 50 employees total), and now it's even bigger. There are hundreds of people working on WhatsApp.
It seems to me that WhatsApp and Vine have a similar staff to user to ratio.
If there are hundreds of people working on WhatsApp I wonder why it's so difficult for them to continue supporting Blackberry 10. So what if it's not popular, they should aim for complete coverage of modern mobile OSes. Then again this is the same team that could not implement encryption without outside help.
I guess serving video is somewhat more complicated than delivering text messages (although texts need to be delivered in nearly real-time).
Maybe staff size follows a logistic curve w/r/t traffic?