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I don't think Onix was ever meant to be as huge as it is. The original sprite doesn't seem to convey that great a size and the designers said on interviews how they weren't the ones to set the pokedex weight/height data on the pokedex so at times they were really surprised how different their official size turned out compared to how they originally envisioned them.
Perhaps this is part of the reason Onix is notoriously weak, because it was not supposed to be the gargantuan snake monster we're used to in the first place.
