They got the look completely wrong
On the cartoon Alvin et al didn't actually LOOK like chipmunks, in the Chipmunk Adventure and the late 80s/ early 90s episodes, they just made them look near-human. It just wasn't a concern for them to look like ACTUAL chipmunks. It was fine that they looked like little people and the audience just accepted it. The A & the C's movie has them look like grotesque rodents and their size isn't to scale as it was in the show which had them human-sized. This looks like Honey I Shrunk the Chipmunks.
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