ADBOC. This is like the efficient market hypothesis, the strong version is wrong and the weak version is so weak as to be almost tautological. In a perfect market the most economically efficient organisation would win, end of story. What we've got isn't that but it's an approximation thereof. The closer it gets to fulfilling all the conditions the more likely it is that the outcome is economically efficient.
The only economy I'm aware of that had a lot of employee owned cooperatives was Yugoslavia. It worked better than Soviet bloc communism as far as resource allocation and living standards went but not as well as capitalism.
Co-ops don't expand as much as privately or publicly held firms because they maximise something approximating profit per worker whereas normal firms maximise straight profits. I think I picked up that argument from Paul Krugman. It's been a long time since I read the argument so I may be misremembering but he used Harvard and the UC systems as examples. Harvard doesn't expand its student body, this maximises faculty utility, the UC system does, this maximises student utility. The article even mentions that worker's co-ops have higher profitability than normal firms.