Giant oil companies are weathering the oil slump better than the average shale driller, but even their famous stability is at times being surpassed by much smaller companies that own some of the choicest U.S. oil-and-gas fields. Take Cimarex Energy Co. XEC 3.33 % , which drills in Texas and Oklahoma. The oil producer’s stock price has fared better than that of Chevron Corp. CVX 0.92 % , the second largest American oil company by revenue, since crude-oil prices started crashing last June. Shares of Diamondback Energy Inc., FANG 4.09 % an eight-year-old oil producer with a $4.3 billion market value, have held up almost as well as Exxon Mobil Corp. XOM 0.80 % ’s over that stretch. And as of this week, Cabot Oil & Gas Corp. COG -1.49 % , a Houston-based shale-gas specialist, had bested all the big oil companies in stock-market performance since last summer […]