A fuel injector is disclosed for use in an engine system. The fuel injector may have a body, a first piston slidingly disposed in the body and having a first pressure area, and a second piston slidingly disposed in the body and having a second pressure area. The fuel injector may also have a hydraulic coupling connecting the first and second pressure areas, and a piezo stack connected to move the first piston. The fuel injector may additionally have a valve needle configured to engage and be moved by the second piston. The first pressure area may be larger than the second pressure area.