The larger an organization is, the harder it is to effectively change its structure. Of course, the basic structure can be easily changed by doing nothing more than putting out a new organizational chart and giving people new titles. But that sort of change is merely cosmetic.
A big organization is hard to change in any meaningful way. The employees of the organization get used to doing things in a particular way. Habits are formed that are geared toward working in the existing structure. When structural change is attempted, the employees have to adjust their expectations and habits. This takes time.
In a smaller organization, rapid change is somewhat easier. This is true simply because there are fewer employees. Fewer changes need to be made to employees' habits. Therefore, it is possible for change to occur more rapidly.
The general answer to this is that the larger an organization, the harder it is to change its structure. The reason for this is that the organization must generally keep one producing whatever it makes even as it restructures. It cannot simply shut down for as long as it takes to change the organization. This means that the organization must change gradually. If it changes too fast, its operations will be thrown into chaos because no one will know how to operate in the new system. Instead, the organization must change relatively slowly so that its members can get accustomed to the changes and can keep producing as the changes occur.