"Our game made €4 million," says Rise of Industry's creator. "Three years later, I was broke"
(www.gamesindustry.biz)
Small team + great game and that's what you get. According to steam revenue calculator, factorio made 83 million net revenue. Probably is quite a bit lower because its using the updated price.
They have like.. 4? People working on that game + accounting and maybe community management. They could go indefinitely.