It comes as fears about the financial market impact if the UK votes to leave the EU next week sent the CBOE Market Volatility Index (Vix), aka the Fear Index, to its highest level in three months yesterday.
"A typical small UK plan is more UK-centric, more exposed to movements in sterling versus other currencies, and is managed in a less dynamic fashion," Nathan Baker, principal in Mercer's investment business, said in Mercer's asset allocation survey.
He continued: "On balance, they would appear more exposed to any associated volatility, and less well positioned to take advantage from it."
However, in a slight to chancellor George Osborne and other Treasury officials, the report also said "it’s not possible to know now with any certainty how the referendum will impact portfolios."
The government has repeatedly warned pension pots will be hit if the UK votes "leave" on 23 June. Osborne previously said such a scenario could leave pensioners £32,000 worse off.
Mercer’s European asset allocation survey includes investment information from nearly 1,100 institutional investors across 14 European countries, reflecting total assets of around €930bn (£739bn).