When Petro-Canada, this country's state-owned oil company, acquired a major interest in Gulf Corp.'s Canadian holdings earlier this month, it was only the latest example of four decades of rapid expansion by government-controlled enterprises.

The $650-million purchase, which catapulted Petro-Canada ahead of Exxon Corp. subsidiary Imperial Oil into the position of the largest service station operator in the country, caused consternation among business people here.

That was because the deal took place under the administration of Prime Minister Brian Mulroney, whose Progressive Conservative Party has long made trimming the scope of government involvement in the economy an article of faith.

Like British Prime Minister Margaret Thatcher, Mulroney wants to sell off to private interests some of Ottawa's massive corporate network, an empire so large that its 300-odd companies were only definitively counted by the federal government a few years ago.

These firms, called Crown corporations, are valued at more than $57 billion and extend to virtually every aspect of the economy, including railways, airlines, the post office, cultural institutions and agricultural marketing boards.

Called a "sub-government" by a federal spending watchdog, they employ more than 250,000 Canadians -- about one worker in 40. An equal number are estimated to work for companies wholly owned by the governments of Canada's 10 provinces.

Like Thatcher, Mulroney sees this enormous government presence in the economy as detrimental to competition and private initiative. But Ottawa has so far taken a more cautious approach to so-called privatization than have Britain's ruling Conservatives.

In one of the first important results in this area since Mulroney took office last fall, the government is expected within the next few weeks to begin divesting itself of its stake in a sprawling holding company, the Canada Development Corp. The company, which with $5.9 billion in assets is the nation's 10-largest firm, was set up with government seed money in 1971 as part of the former Liberal Party administration's efforts to enhance Canadian ownership of the nation's leading industries.

Over the years, the company has amassed an impressive string of petrochemical, biotechnology, mining and energy businesses. Among its most publicized ventures was a $3 billion buying spree in 1981 that included the takeover of Savin Corp., a U.S. office copier maker, and the Canadian properties of Texasgulf Inc. of Stamford, Conn.

From a wholly owned government property, the CDC has over the years evolved into a corporate hybrid, held partly by Ottawa and partly by private investors.

Ottawa plans to offer its entire 47 percent ownership (30.7 million common shares) for sale, in two stages, by the end of the year. Stock market analysts generally expect a positive response.

For the first time, foreign interests will be allowed to invest in CDC, up to a maximum of 25 percent ownership, although no single foreign investor can hold more than 10 percent.

"This sale reflects our view that the government should not be involved in the ownership or operation of commercial enterprises which perform no public policy role," said Industry Minister Sinclair Stevens when the sale was announced earlier this summer.

Among the other federally owned enterprises up for sale are uranium producer Eldorado Nuclear Ltd., international telecommunications company Teleglobe Canada, armaments manufacturer Canadian Arsensals and two debt-plagued aircraft manufacturers, De Havilland and Canadair.

To aid in the search for buyers, a group of international investment experts -- including Burns Fry Ltd. of Toronto, Merrill Lynch Capital Markets of New York and Warburg and Co. Ltd. of London -- has been assembled.

"Selling businesses is not easy," said John MacNaughton, director of Burns Fry. He said the main impediment is the size of the commitment required from potential buyers.

The two aircraft makers have been forced to turn to the government for financial rescue packages in the past three years. De Havilland has lost $200 million since 1982.

But MacNaughton said negotiations were continuing with a number of potential buyers and expressed confidence that deals may be reached fairly soon.

While Ottawa is committed to privatization, the Mulroney government has balked at the sweeping approach of the current British government. "I'm not opening a store," said Treasury Board President Robert De Cotret.

Rather than produce a long list of corporations to be auctioned off whole, as in Britain, the Canadian government will move slowly, examining each company and each offer on its merits, said De Cotret, who is one of the key cabinet ministers involved. Behind this thinking are concerns about the lack of available capital in Canada and the need to limit foreign purchases of Canadian assets, federal officials said.

But business people have long complained about the size of the "public sector" in Canada and the drag unprofitable Crown corporations exert on Ottawa's budget, now running an annual deficit of $25 billion. So the lack of results to date has inspired doubts about Mulroney's '84 campaign promises to reduce the size of government.

"People in the private sector are looking to the government to get on with privatization because it was an important part of the Conservative message," said Roger Hamel, president of the Canadian Chamber of Commerce. "We do get the odd conflicting signal," he added, referring to the government's decision to allow Petro-Canada to buy Gulf Canada's marketing and refining operations early this month.

Created in 1971 by a minority Liberal government dependent on the socialist New Democratic Party for its existence, the Crown-owned oil firm has, through a series of acquisitions, grown to become one of the top oil and gas conglomerates in Canada. It has assets of $7.5 billion, extensive oil and gas reserves and a prominent position in drilling exploration on Canada's frontiers, including the promising Hibernia offshore oilfield near Newfoundland.

Conservatives, as the voice of private business and western Canada's oil-producing region, aired bitter criticism over the years as Petro-Canada acquired Atlantic Richfield Canada, Pacific Petroleums Ltd., Petrofina Canada Inc. and part of British Petroleum's Canadian operations.

Mulroney government spokesmen said the recent Gulf purchase was acceptable because it dovetailed with the Conservatives' goal of broadening Canadian ownership of the petroleum industry. The current government still adheres to this objective, although it has rejected much of the nationalistic energy policies that put former prime minister Pierre Trudeau at loggerheads with oil companies and the U.S. Congress in the early 1980s.

Many industry analysts speculate that Ottawa allowed the Gulf deal for another reason: to improve Petro-Canada's retail network in order to make it attractive to investors should the government decide to make shares in the company available to the Canadian public in the next few years.

The government had also been expected to consider partial privatization of Air Canada, the debt-plagued national airline. But Mulroney, under pressure from unions worried about the effect such a move would have on job security, said early this year, "Air Canada is not for sale."