Vent Pipe Undersized

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I recently discovered that the vent pipe from my kitchen sink and dishwasher is undersized. The contractor routed what looks like 1" pipe through the wall (presumably because there wasn't enough room to route 2" pipe), which then feeds into a 2" pipe that leaves the house. There is no transition/connector piece between the 1" pipe and the 2" pipe. The 1" pipe simply sits inside the diameter of the 2" pipe. The "connection" point is about 2 feet below the roof. I am able to see this only through the hole in the roof as all piping is behind the wall. To keep out debris and rain, the contractor also added a 180 degree elbow to the top of the 2" pipe through the roof. I believe per code, the 1" pipe and elbow are both not permitted. I am wondering what I am risking by maintaining this setup. Why is 1.25" vent pipe the minimum allowed per code? Is there conservatism built into the code? It seems that any dangerous gasses would still be able to exit the house, although if obstructed by the small diameter pipe or elbow heavy particles may fall back into the house behind the wall?
Any insight is greatly appreciated!
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