It's difficult for a giant to move quickly.

The nation's biggest stock funds have held their own in 1991, but many of their lumbering blue-chip portfolios have failed to provide the zip that more diminutive funds investing in small-company stocks offer.Conservative investors, who seek out these massive funds for talented management that protects them from negative surprises, should find the latest double-digit returns perfectly acceptable.

But they shouldn't expect any hefty 30 percent gains.

Among the biggest, $15.2 billion-asset Fidelity Magellan has turned in a strong 20.23 percent gain, thanks to the fact that portfolio manager Morris Smith holds many small-company stocks. That's unusual for a big fund.

Meanwhile, $7.6 billion-asset Windsor Fund has managed an 18.25 percent gain, reaping the rewards of portfolio manager John Neff's steadfast belief in bank and insurance company stocks.

Is it difficult to be nimble when your fund is elephant-size?

"The big mental change in portfolio management is moving from $500 million to $1 billion in assets, but beyond that point there's not a big difference," explained Beth Terrana, portfolio manager of the $4.1 billion-asset Fidelity Equity-Income Fund, who last October left the $2 billion-asset Fidelity Growth & Income Fund after five years at its helm.

"I own 300 names in my portfolio, and it's a lot of work to track them, but I do have help from more than 60 analysts and Fidelity's other fund managers."

Terrana's fund is up a creditable 13.86 percent, emphasizing inexpensively priced, well-known companies. Its growth-and-income goal, like many of the superfunds, is to beat the market by 1 or 2 percentage points.

For example, she holds stock in K mart because its management is gradually improving the company; Entergy Corp. because of its huge cash generation and its steps to buy back stock and increase dividends; and Continental Corp. since it's the insurance firm most hated by Wall Street, even though its top brass is making the right moves.

Other major holdings include Texaco because it's poorly perceived but attractively priced, and First Chicago Corp. since chairman Barry Sullivan is doing a much better job of running the bank than the investment community is currently giving him credit for.

"The big funds don't offer the romance of a flashy small fund, but they don't provide ugly losses either," said Don Phillips, editor of the Chicago-based Mutual Fund Values advisory service. "With the tide turning toward smaller-company stocks, the small funds will gain assets more rapidly, but remember that the reasons these well-known funds are so big are the quality of their managers and their long-term performance."

According to Peter Langer, a senior vice president with the Capital Research & Management Corp. that manages the nation's third-, fourth-and ninth-largest funds, the fact that aggressive funds are doing better right now doesn't mean the most-popular funds will lose steam with the public.

"So long as the investor puts money in the fund regularly and realizes the cyclical nature of the market, long-term results will be impressive," said Langer. "We still see money pouring into our more conservative funds at a record pace."

Here, ranked in order of asset size by Mutual Fund Values, are the nation's biggest stock mutual funds and their first-quarter 1991 performances:

Fidelity Magellan Fund, Boston; $15.2 billion in assets; 3 percent "load" (initial sales charge); up 20.23 percent.

Windsor Fund, Valley Forge, Pa.; $7.6 billion; no load, but currently closed to new investors; up 18.25 percent.

Investment Co. of America, Los Angeles; $7LECKEYContinued from D?

billion; 5.75 percent maximum load; up 12.12 percent.

Washington Mutual Investors, Washington, D.C.; $6.4 billion; 5.75 percent load; up 10.89 percent.

Fidelity Puritan Fund, Boston; $4.7 billion; 2 percent load; up 11.27 percent.

Fidelity Equity-Income Fund, Boston; $4.1 billion; 2 percent load; up 13.86 percent.

Pioneer II, Boston; $4.1 billion; 8.5 percent load; up 11.71 percent.

Templeton World, St. Petersburg, Fla.; $4.1 billion; 8.5 percent load; up 14.67 percent.

American Mutual, Los Angeles; $3.7 billion; 5.75 percent load; up 8.09 percent.

Twentieth Century Select Investors, Kansas City, Mo.; $3.7 billion; no load; up 12.68 percent.