As a project manager, your biggest responsibility is to keep an eye on whether a project's scope increases in ways you haven't planned, which is called scope creep. When you do a good job managing scope creep, you make sure that the project won't outgrow its resources, for any reason. If a project grows too big, or too fast, the project has a high chance of failing. Usually, when a project has grown too big, it's because the project's resources have become unmanageable. That's scope creep.