Whatever the world’s economic and market turbulence last year, one group has held up well: billionaires.

The combined wealth of the world’s billionaires, defined as individuals with a net worth of $1 billion or above, increased by 5.4% to a record $7.7 trillion, according to Wealth-X’s 2015-2016 billionaire census. By comparison, the gross domestic product of the U.S. is around $17 trillion.

The...