THERE'S less sizzle - and more fizzle - in the $75 billion fast-food industry these days, as the major chains find that they overextended themselves in the 1980s.If it often seems as though the United States is awash in fast-food outlets, the reason is simple: It is. From the Pacific to the Atlantic, fast-food stores can be found along most major highways. The "industry is overbuilt and maturing," concludes Mark Manson, an analyst with Donaldson, Lufkin & Jenrette Inc. Mr. Manson sees only "winners and losers" in the industry; winners, he reckons, are chains that can increase market share. Operating on minuscule profit margins, with revenue growth largely stalled within the domestic US market, some chains are now finding it necessary to slash prices - and hence profits - just to maintain existing market positions. "There's still room for growth for the restaurant industry within the US," says Michael Mueller, an analyst for Montgomery Securities Inc. Yet, for many of the older chains, he says, "there's no price flexibility left," to enable the chains to slash costs and turn in respectable profits. The main opportunities for investors, Mr. Mueller says, involve the smaller, lesser-known "speciality" chains, such as Shoneys, Ryans, Sizzler Steak House, Au Bon Pain, and the Spaghetti Warehouse. Many of these chains, he says, "specialize in the new demographic wave," the "graying of America," and the maturing of the baby boomers, who in the 1970s and '80s gave momentum to such front-runners as McDonald's. And for all its current turmoil, such as ruthless pricing wars, the restaurant industry continues to win plaudits from many stock market strategists. Dennis Jarrett, a technical analyst for Kidder, Peabody & Co., for example, sees the restaurant industry as one of the leading stock sectors in terms of market momentum. That said, there's little disagreement here that growth is slowing and competition is becoming intense within the fast-food segment of the restaurant industry. McDonald's, for example, is increasingly dependent on overseas sales, notes Mueller. The chain recently reported record profits for the third quarter of 1991. Yet, the report also showed weakness in two key overseas markets, Canada and Britain, Mueller says. McDonald's is still the king of the fast-food hill; but its share of the overall US market has slipped from about 19 percent in the mid-1980s to about 17 percent in 1991. To maintain leadership, McDonald's has been cutting prices (offering a 69-cent cheeseburger) and turning to a more diversified menu (including breakfast burritos). McDonald's is also considering an evening dinner menu aimed at adults and families. But moving to a more diversified breakfast/dinner menu will require substantial "public education," says Mueller. That, in turn, will require substantial advertising dollars. McDonald's, experts say, will probably be forced to continue to broaden its menu, given intense pressure from its toughest competitor, Pepsico Inc. Pepsico owns three fast-food chains, Taco Bell, Pizza Hut, and Kentucky Fried Chicken. Pepsico now accounts for about 13 percent of US fast-food sales and is growing. Last week, Taco Bell began offering low-cost breakfast items. Although the breakfasts are currently limited to the Los Angeles area, Pepsico plans to go national with the fare by next summer. Taco Bell's morning prices are now below McDonald's on some items. Whether McDonald's makes further price cuts or not, other chains will find it difficult to avoid slashing prices on their own products if they want to remain competitive. Wendy's, for example, has been doing that. Finally, Pepsico still trails its own main rival, Coca Cola Company, in terms of overall growth, although both stocks are well-regarded. Coca Cola is primarily a beverage company. Pepsico is highly diversified, and more dependent on the US market. Coca Cola's growth is linked to strong sales abroad, where it dominates the soft-drink market.