Sharp discounting and cold weather helped make Christmas a far brighter sales season than retailers had expected, according to figures released yesterday by the nation's major chains.

"It was hardly the bust people were expecting" after the stock market collapse Oct. 19, said Monroe H. Greenstein, a retail industry analyst with Bear, Stearns & Co. "It shows once again that if people spend one time of the year, it will be for Christmas."

The December sales figures varied greatly from chain to chain, with the nation's largest retailer, Sears, Roebuck & Co. reporting one of the smallest increases -- 4.9 percent over December 1986 sales -- while the discount chain Wal-Mart Stores Inc. posted a 34 percent increase in sales.

The numbers "are a little bit stronger than a lot of people had expected, thanks to chilly weather and fairly significant promotional activity as well," said Edward A. Weller, an analyst at Montgomery Securities. Consumer spending -- which has been fueling the economy for the past few years -- "for now appears to be moving up, but it remains spotty," Weller said.

However, retail executives said, the gains in sales came at the expense of the bottom line, as most retailers discounted heavily to sell their goods.

"There is much less here than meets the eye," said William N. Smith, an analyst at Smith Barney, Harris Upham & Co. Retailers "got the sales they wanted -- they bought them," he said.

As a result, some retailers yesterday said their profits took a beating during the Christmas season. J.C. Penney Co. Chairman William Howell, for example, said that even though the chain had an 8.6 percent sales increase in December, Penney's earnings for November and December were flat compared with last year because of the heavy promotional environment.

The sales gains also were aided by the opening of new stores, figures showed. For example, although Dayton Hudson Corp. reported a 19.1 percent sales increase in December, stores that had been open for a year or more reported only a 1.4 percent gain. Similarly, although the Limited Inc. reported a 7 percent gain for the month, the specialty retailing chain had a 2 percent sales drop in stores that were open a year or more.

Greenstein said the sales figures showed that expensive items did better than expected.

Many retailers had been predicting a slowdown in the high-ticket items because of the stock market collapse. But Neiman-Marcus Group Inc. rang up a 9 percent increase and Tiffany's posted a 21 percent increase, Greenstein said.

Specialty apparel chains such as The Limited, on the other hand, continued to have disappointing sales, partly because of the confusion over fashion trends, the shift toward dressier clothes and price increases on imported goods, which account for a larger percentage of merchandise in specialty stores than in department stores.

Among the other chains issuing sales results yesterday: K mart Corp., the nation's second largest retailer, posted an 8 percent sales gain in December, with stores open a year or more recording a 5.7 percent gain.

May Department Stores Co., the parent of the Hecht Co., had a 10 percent increase in sales for the Christmas season, with sales from older stores increasing 6.5 percent.

Federated Department Stores Inc. sold $1.9 billion in December, a 5.2 percent increase over a year earlier.

Carter Hawley Hale Stores Inc. sales rose 3.1 percent to $481.5 million.

F.W. Woolworth Co.'s domestic sales increased 10 percent to $687 million. The gain "was the strongest since 1983 and the second best in the last nine years," said Woolworth Chairman Harold E. Sells.

Montgomery Ward & Co.'s overall sales rose 5.4 percent to $757.7 million; stores open a year or more recorded a 3 percent gain.

Best Products Co. Inc. saw sales rise 3.8 percent for the month to $491.5 million.

Bradlees Discount Department Stores rung up a 10 percent increase with sales $397 million.