Got it, I think it's a solo developer who has been going for approx 500 updates, 1 week 1 update.
I think it's two developers, a programmer and an art guy. These are also the guys behind the Humble Bundles, so I imagine quite a bit of their time gets sucked up by that.
Overgrowth is a great demo, but imo not worth the price, unless you really loved their first game, or just want to support an awesome indie company. For EU guys if you do plan on buying it, get it on
http://www.wolfire.com/overgrowth, rather than steam. 25 dollars is a lot less than the 24 euros it costs on steam and you get a steam key anyway.
I also want to recommend one of Wolfire's other games, receiver. That game has the most interesting and engaging gun mechanics. It's certainly worth picking up in a steam sale some time (or you get it for free if you buy overgrowth)