Standard & Poor's 500 Index companies that have filed proxy statements for fiscal 2014 contributed an average $1.5 million to their CEOs' pensions, compared to an average of about $550,000 in 2013, according to summary compensation table data compiled by Bloomberg. Pensions comprised about 11 percent of total CEO pay at those companies, compared to 4 percent in 2013.

The ballooning pensions were often triggered by preset agreements on CEO retirement pay, rather than new decisions by boards to give their executives more. Longer lifespans and lower discount rates are forcing companies to add more to their CEOs' pensions to meet return expectations and support future payouts, said Steve Seelig, an executive compensation consultant at Towers Watson & Co.

"This whole question about pension values is coming up simply because it's rising so much due to things that aren't really design related," Seelig said.