Titan is a moon, not a planet. That means Titan does not orbit the Sun by itself, but goes around its planet Saturn. Most planets of the Solar System are bigger than most moons. The answer lies in the fact that Mercury and Pluto are very small planets and Mercury is a very big moon. Even though they are small bodies, Mercury and Pluto orbit the Sun, which makes them planets.
Titan is so large that it even has clouds and an atmosphere.
In fact, it probably is the only moon in our Solar System with a significant atmosphere. Also, we need to breathe oxygen and there is none in Titan's atmosphere. The clouds surrounding Titan are so thick that even the Voyager spacecraft, which flew quite close to the moon, could not see down through the clouds to the moon's surface. So, we don't know what the surface of Titan looks like, although we think it must be very cold. Titan could have mountains and oceans, but we won't know for sure until our probe visits.