Small charities are important, but their net impact on the country is often small. An established and powerful NGO has the ability to reach a larger portion of the population. Their ability to lobby, finance, and implement policy and change have a greater effect in the long run.
Additionally, they are often much easier to donate to. They typically have easy to navigate websites with simple donation processes. Smaller organizations may require you to physically donate or contribute in some way. The relative ease of supporting a large NGO ultimately means that more people will back them.
Lastly, the larger NGO's require a much higher funding threshold than a smaller organization. If a larger group is better funded, they can likely better meet the needs of someone than a smaller organization.