Factual error: As a trash compactor Wall-E does not function logically. When he fills his chest compartment with garbage and runs his internal compactor, the cube that exits his body is the same volume as the trash he puts in, despite that trash having been compacted. He does not add extra trash to fill the empty space after running the compactor - there are three scenes that show him filling up only once with loose garbage and then ejecting a densely compacted cube. (00:02:30)
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Suggested correction: You haven't noticed the exact shape of his body. The back sticks out a tad. Garbage fills the space and the back pushes in to compress it.