Notes

This doesn't help anyone (there is more trash than he could get through in a million lifetimes, all the other trash compactor robots are dead, and the humans aren't even around anymore). So right from the beginning of the movie we are shown a [wordlessly] absurd situation. But Wall-E keeps working because he lacks the ability/initiative to question his existence until he gets that spark from EVE. Later aboard the AXIOM he meets a bunch of other robots programmed to do silly or futile tasks. The robots go rogue because they eventually understand there is a higher calling than duty. Meanwhile, the humans have wrecked Planet Earth because each human being was passively fulfilling their role/duty as a consumer without thinking about larger consequences (a sort of "No drop of water thinks it's the ocean" situation). When the humans run away aboard the AXIOM, they still continue their lives as passive consumers. They don't even lift a muscle to consume food and entertainment. People think the message is "The AXIOM is a dystopia because everybody's fat" but it's actually "The AXIOM is a dystopia because nobody is thinking about the purpose of their lives." They are letting the machine of society, in this case a literal conveyer belt, shuffle them along a consumer lifestyle. Everybody being obese is only one more way the movie hammers home that everyone has become passive.

In the end of the movie Wall-E literally fulfills his purpose. He was made to clean up the earth and he ironically does that by going rogue, leaving his task, and bringing the humans back. So the true theme of the movie is something like "You have to break out of your rut to really live."