I think many interventions initially face increasing returns from learning/research, creating economies of scale, specialization within the cause area, etc. For example, in most cause areas, the first $10,000 are probably invested into prioritization, organizing, or (potentially symbolic) interventions that later turn out to be suboptimal.
(I strongly recommend this neglected (!) article.)
Economies of scale are a force for increasing returns, and they win out while still at a small scale, so the impact of the 5th staff member can easily be greater than the 4th.
Economies of scale are caused by:
1. Gains from specialisation. In a one person organisation, that person has to do everything – marketing, making the product, operations and so on. In a larger organisation, however, you can hire a specialist to do each function, which is more efficient.
2. Fixed costs. Often you have to pay the same amount of money for a service no matter what scale you have e.g. legally registering as an organisation costs about the same amount of time no matter how large you are; an aircraft with 100 passengers requires the same number of pilots as one with 200 passengers. As you become larger, fixed costs become a smaller and smaller fraction of the total.
3. Physical effects. Running an office that’s 2x as large doesn’t cost 2x as much to heat, because the volume increases by the cube of the length, while the surface area only increases by the square of the length. A rule of thumb is that capital costs only increase 50% in order to double capacity.
Caspar Oesterheld makes this point in Complications in evaluating neglectedness:
(I strongly recommend this neglected (!) article.)
Ben Todd makes a related point about charities (rather than causes) in Stop assuming ‘declining returns’ in small charities: