In my experience (and I generally have found myself in the position of 'person that has the system in their head'), it's not necessarily about size but about being able to sit down and work with the system. It's not just about having the theory but also having the time actually working with the system and learning how things act in practice, which is a kinda weird mix between being a senior role with a high-level view of the system and a theoretically quite junior role of more or less technician.

(It also usually involves some quite proactive learning of the form of finding the specialist in some part of the system and sitting next to them and going "explain to me how this part works", and then repeating more or less indefinitely)