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Extra weight (because all the extra metal you need to have curved plares)
Wears faster (small pins all the way along the chain rather than every other)
Not as efficient (two above combined)
Basically worse in every way important, except for grinding on (because of all the extra metal) and giving slightly more adjustment, which you probably don't need as most dropouts are long enough to take way more than half an inch adjustment.
Is the chain new?
If you can set it up (smaller tyre) the first few miles will probably stretch the chain (not real stretch, more like settling in cereal, pushing gaps out!) enough to put the old tyre back.
This may only work with cheap chains?