Santa Monica, California-based consulting company Wilshire Associates Inc said public pensions in the US posted median gains of 16.1% in 2013, marking the fourth time in the last five years that they reported returns in double digits, Bloomberg reported.

According to the Wilshire Associates report released today, the smaller public pensions that put more of their assets in US stocks were able to outperform the plans that have assets amounting to over $5 billion. For the first time since 1999, the benchmark Standard & Poor's 500 Index of US stocks ended 2013 on a high note with a 30% increase.

Wilshire Associates Managing Director Bob Waid told Bloomberg, "U.S. equities trumped all other asset classes."

Citing data from the US Census Bureau, the report said that assets of the 100 biggest public pension funds in the US increased to $3.06 trillion in the third quarter of last year. This compared with the $2.94 trillion in the previous quarter and represented the highest rise since 1968.

Wilshire Associates added that state and local government funds that have assets of less than $1 billion posted a median return of 16.45%, beating much bigger plans that only gained 15.76%. Wilshire's Trust Universe Comparison Service showed that state and local pensions nationwide allocated an average of 45.3% on US stocks, the report said.

On the other hand, mega public pensions that hold assets of over $5 billion had a 36.4% median allocation in US equities. Their median allocation for alternative assets was also much larger at 12.1% when viewed side by side with the 0.73% allocation for all public pensions. These alternative assets included private equity and hedge funds, the report said.

In an interview, Waid told Bloomberg, "The larger plans have been really good about diversification. That, in theory and observed in practice, works good over the long term, but you'll have short time periods where if you're overweighted in one asset class and that asset class does really well, you're going to do really well."