They're not planet-sized. Wiki says one is about 300m and another 1.2km. Everything in an orbit is traveling the necessary speed for that orbit, otherwise it wouldn't be an orbit. In most cases, an object in the same orbit around the Sun as Earth would eventually be pulled into a different orbit by Earth's gravity, but these asteroids are moving around a Lagrange point where the gravity of the Earth and the Sun kinda cancel out for orbital purposes.