Large organisations often use consultants from the start to help manage change and major projects. They recognise that making major change is stressful from a rational perspective i.e. doing the things that need to be done but also an emotional perspective i.e. managing people through change especially if they are also affected by the change. Poorly executed change can not only fail to deliver the benefits of that particular change, but affect the rest of the business.
Smaller organisations tend to rely on internal resources