The combined assets of the world’s largest 300 pension funds grew more than 3 percent in 2014 to a new high of more than $15 trillion, driven by retirement saving in North America and Europe, a study showed on Monday. North America had the highest five-year combined compound growth rate of around 8 percent, according to the study, compiled by specialist newsletter Pensions & Investments (P&I) and consultants Towers Watson.