The average compensation package for CEOs in the U.S. rose nearly 13 percent over the past year, driven by increasingly valuable pension plans, according to new research that that analyzed early filers in the Russell 3000 index.

The study, conducted by two consultancy offshoots of the proxy advisor Institutional Shareholder Services (ISS Corporate Solutions and ExecComp Analytics), looked at companies that reported their financials through April 13, 2015. That pool encompassed more than one-third of Russell 3,000 companies (1,211), limiting the data set to CEOs who have held their post for at least two years.

Nearly two-thirds of those CEOs enjoyed a pay increase for fiscal year 2014, a two percentage point increase over the previous period. CEO pay, the study says, jumped 12.7 percent, fueled largely by actuarial increases in the value of executive pensions. Among the sampled companies, average CEO total grant-date pay (including changes in pension values) totaled $6,433,296, compared with $5,538,002 for the previous year. Excluding pension changes, the median increase in total CEO compensation was 7.2 percent.

Among firms that use equity compensation, the median grant date value of stock awards—most of which were allocated in late 2013 or early 2014—increased by 11.9 percent, due to strong 2013 stock market performance.

Connecticut CEOs saw the largest median pay increases at 38.7 percent, followed by those in the San Francisco area (19.5 percent) and Washington, D.C. (15.4 percent). The 98 New York City area corporations that have filed financials thus far in 2015 netted a median increase of 12.3 percent year-over-year. 

The new data feeds into an escalating debate over the gap between executive compensation and worker pay. Public companies currently await a final rule from the Securities and Exchange Commission that requires them to disclose a comparison of their CEO's pay to that of the median employee in their global workforce.

Also, on the first day of her official presidential campaign, Democrat Hillary Clinton seized upon the growing scrutiny of CEO pay, pitting middle and lower class economic hardships against “the average CEO [who] makes about 300 times what the average worker makes." Clinton’s sound bite drew upon research by the Economic Policy Institute, a self-described “nonprofit, nonpartisan think tank” focused on wage inequality. It claims that CEO pay has increased from approximately 20 times an average employees pay in 1965, to the current 300-1 ratio.

While business leaders may credit the compensation jump for CEOs on a recovering economy and buoyant stock market, those gains haven’t translated into increased wages. According to February numbers from the U.S. Bureau of Labor Statistics, average hourly earnings for workers have only increased 2 percent over the past 12 months, a percentage increase that has held relatively steady for the past five years.