So the other 60-70% of GDP is produced by one-third of the population?

If I said 30%-40% of GDP was produced by a certain set of companies, and two-thirds of the population were shareholders in those companies, would that not seem to add up?

A cooperative member is not a shareholder in the sense investor-expecting-return, but many people may take out membership as consumers, or simply to support the coop. They are not necessarily actively engaged in production.