I've run some numbers to compare the biggest U.S. carriers, and it reminded me how difficult it is to definitively say who's the biggest.

If you take revenues and expenses, the merged Continental Airlines and United Airlines would be larger than all its U.S. peers.

But if you take fleet, traffic and capacity, Delta Air Lines would be largest.

Here's how it goes by category:
• Operating revenues: The combined United and Continental, at $28.9 billion
• Operating expenses: United-Continental, $29.2 billion
• Operating income: Southwest Airlines, $262 million
• Net income: Southwest Airlines, $99 million
• Full-time equivalent employees: United-Continental, 82,340
• Airplanes flown by the mainline carrier, not regionals: Delta Air Lines, 740
• Revenue passenger miles: Delta, 188.9 billion
• Available seat miles: Delta, 230.3 billion

Source: AIRLINE BIZ Blog | The Dallas Morning News