The record $3.5 trillion stockpiled in U.S. fixed-income funds is raising the specter the bond market will buckle under the strain of redemptions once interest rates rise. Simple math suggests there's little need to worry.

While fund assets have almost doubled since the financial crisis, their size as a portion of all U.S. mutual funds is still in line with the 21 percent average over the past three decades, according to Investment Company Institute (ICI). Debt fund managers also hold almost 9 percent of assets in cash—close to the highest in a quarter-century and more than the maximum rate of outflows in 1994, one of the worst years for bonds.

With St. Louis Fed President James Bullard saying Aug. 15 that markets are probably mistaken if they expect rate increases to occur more slowly than policy makers forecast, money managers such as BlackRock Inc. have called on regulators to forestall a potential liquidity crisis that may deepen a selloff if too many investors try to get out at once. While a tumble in bonds economists have predicted all year may finally materialize, JPMorgan Chase & Co. says withdrawals are unlikely to unleash a flood of selling that upends debt markets.