> Valve maybe choosing to stay small to avoid scaling beyond their effective structure.

I wouldn't call 300 people a small number for the size of a company.

In fact, Jeri Ellsworth addresses this point (I'm quoting the transcript by develop-online [0]):

"Their structure probably works really well with about 20 people, but breaks down terribly when you get to a company of 300 people. Communication was a problem. That's where management… Well if I had anything to do differently, would be to make sure a layer of management could do communication correctly."

So it looks like Valve has already scaled beyond their effective structure.

[0] https://www.develop-online.net/news/44746/Valves-perfect-hir...