Looks like a partial collar error. It occurs when the unstruck planchet is not seated IN the collar, but ON top of the collar. The striking pressure results in the coin being struck literally, partially in the collar, and the smaller diameter of your coin (the reverse in this case, but not in all cases with modern US coins) is the side that was IN the collar. The broader edge was the part of the planchet that didn't get into the collar, and is slightly broadstruck. These are also called "Railroad Rim" errors, because they look like the edge of a train wheel.
