You can't just vaguely communicate facts to a small set of team members and expect the facts to naturally disseminate among the rest of the group. You have to learn how to communicate quite explicitly. As a group increases in size, you have a whole slew of management challenges. Communicating badly exponentially increases the possibility of making fatal mistakes. A large-scale project has a lot of moving parts, which makes it that much easier to break down.
Communication is the oil that keeps everything working properly.
It's much easier to address an atmosphere of distrust among a group of five team members than it is with a team of 500 members. A project with 100 team members is 20 times bigger in terms of team members, but it's astronomically more complex in terms of communicating. Add to that the fact that most large systems have multiple sites and are implemented in phases, and you have even more complications. With large-scale projects, it's best not to think only in terms of one project manager. I use a model made up of four sets of activities when assessing large-scale projects.