Behind Wells Fargo & Co.’s ascent to the world’s most valuable bank lies a cheap and reliable source of funds favored by regulators and less popular with its largest rivals: retail bank deposits.

Wells Fargo gets 78 percent of its funding from deposits, and 90 percent are so-called core deposits—small amounts from individuals and other account-holders that are viewed as slower to yank their money than big institutions, data compiled by Bloomberg show. The bank has more than tripled deposits in seven years, adding the equivalent of Atlanta-based SunTrust Banks Inc. in the last two.