Several major European banks were found to hold assets that dwarfed the gross domestic product of their country. These were UBS and Credit Suisse in Switzerland, ING and Rabobank in the Netherlands, Santander in Spain and HSBC in the U.K.
(Read more: 'Fat finger' error sees HSBC shares spike 10%)
"European banks—not only in the periphery—are too large for governments to bailout," RBS analysts led by Alberto Gallo said in a report.
Perhaps unexpectedly, Denmark was the country with the biggest bank in comparison to its economy. Denmark's Danske Bank holds assets of 3.3 trillion Danish krones ($597 billion), almost twice the size of the country's economy.
(Read more: Danish government in peril over Goldman-led deal)
Meanwhile, Switzerland's UBS and Credit Suisse have assets worth 174 percent and 148 percent of the Swiss economy.
The U.S. came off better: JPMorgan Chase, Bank of America Merrill Lynch, Citi, Goldman Sachs and Morgan Stanley each hold assets that amount to less than 20 percent of U.S. GDP.
RBS did not say how big its assets were in comparison to the U.K. economy.
—By CNBC's Katy Barnato. Follow her on Twitter @KatyBarnato