What is the animated movie with 2 city sized giant robots?

I remember watching an animated (not cartoon) movie on TV around 2003 maybe? where its a post apocalyptic world where most people lives in decaying city on the back of a giant robot and they fight against people who dont live in the robot because there simply isnt enough space. But this girl ventures out and finds a potential 2nd robot that could take everyone off the planet before the earth blows up! anybody know this movie? ***** Spoiler Alert******* I also remember at the end the two giant robots fight and they are so big that their heads pop out from the clouds and the girl is in the 2nd robot and she controls it buy submerging herself in a pool with stars and she 'Becomes' the robot and rips the city off the back off the older robot which is being controlled by the bad guy and takes everyone to safety...