As crude prices tumble, big oil companies are confronting what once would have been heresy: They need to shrink. Even before U.S. oil prices began their summer drop toward $80 a barrel, the three biggest Western oil companies had lower profit margins than a decade ago, when they sold oil and gas for half the price, according to a Wall Street Journal analysis. Despite collectively earning $18.9 billion in the third quarter, the three companies— Exxon Mobil Corp. , Royal Dutch Shell PLC andChevron Corp. —are now shelving expansion plans and shedding operations with particularly tight profit margins.