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back pressure in exhaust is a bad way to put it, exhaust systems fallow the same rules as wind interments, the cold air could come back up the tail pipe because the exhaust is a pulse and to large of a tail pipe is making go out of tune and creating weird waves echoing back.
I have no heard of modern cars having the problems you speak of due to over sized exhaust, but over sized exhaust tends to do more harm then good performance wise unless the engine is running at higher revs then it was designed to run at under normal use leaving it to run poorly at normal speeds, it is however a problem if you have a cracked exhaust manifold that is letting a cold draft hit the valves, cold air will bend them.
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