As Delta Air Lines settles into its new role as world's largest carrier, it is coming up against a critical question: How big is too big?

In recent months, Delta has sought approval for deals with competitors that would expand its market presence in New York, Australia and in Japan --- and in some cases run into significant opposition.

Delta became the world's biggest airline through its merger with Northwest Airlines, which closed in October 2008 but is still in progress from an operational standpoint.

Before that, Delta had been the No. 3 U.S. airline, with the Southeast as its traditional stronghold. United and American airlines were bigger.

Delta got Justice Department approval of the Northwest merger under the Bush administration. But observers say the Obama administration is likely to exercise greater scrutiny of cases where Delta is seeking approval for agreements with competitors.

A vocal opponent of Delta's merger with Northwest, House Transportation Committee chairman Jim Oberstar (D-Minn.), "is certainly concerned about lack of competition and effects to service as we have more and more consolidation in the airline industry," said spokesman Jim Berard.