Chrysler Corp. Chairman Lee Iacocca's 1988 compensation dropped by nearly 80 percent to $3.66 million because he received only a fraction of the money he did in 1987 from cashing in stock options, the company said. It was a far cry from 1987 and 1986 _ when Iacocca received $17.9 million and $20.6 million respectively _ but still enough to prompt the United Auto Workers to issue a damning statement about "the annual executive pig-out." The drop also raises the possibility that Iacocca, for the first time in three years, wouldn't win the auto executive pay sweepstakes; for 1987, he outdistanced his closest competitor, Ford Motor Co. vice chairman Harold Poling, by $7 million.