Note that the paid staff in the top 400 charities in the UK earned the same as they would have in the private sector. You can understand why. In a charity like Oxfam with a voluntary income of £45 million and a total income of £62 million and net assets of £28 million, in 1992, one can understand the need for paid, full-time, management.
In 2016, Stanford University discovered the same thing. The Stanford University research is important because it reaffirms the need for charities to understand that they can be run by paid staff and also that funders are aware that this is a must for the charity to have long lasting impact.