Mitsubishi International Corp. and Dexim Inc. are both international traders, but the Japanese company, headquartered in New York, moves more than $10 billion in goods and commodities into and out of the United States annually, while the volume of Concord, Mass-based Dexim is closer to $3 million.

Mitsubishi and other Japanese trading companies are large because that island nation traditionally has had to trade in order to survive. Its laws and institutions have been shaped to promote trade.

Dexim and most other U.S. trading companies are small for the opposite reasons. In fact, Americans don't "trade" much at all. U.S. trade companies either import or export, but few do both.

Japanese trading companies enjoy a close relationship with their banks, which gives them two advantages. First, they have access to the capital they need to finance their own business. Dexim's president, John Deegan, says that he turns away 25 to 30 proposals every year, in part because he doesn't have the capital to support additional trade.

The second advantage Japanese trading companies have by virtue of their close bank relationship is an ability to make equity investments in the companies from which they buy. This helps their suppliers to grow, and it cements the relationship between the trading company and its source. Dexim can't afford to invest capital in its sources, which means that as a company's export volume grows there is nothing binding it to Dexim.

"We've taken several companies up to the point that they were paying us $300,000 per year," says Deegan, "only to have them decide they didn't need us anymore."

Another factor that has kept U.S. trading companies small has been the product, rather than market, orientation of most U.S. manufacturers. Americans have traditionally looked at international trade from one perspective: "This is what we make, now where can we sell it?" The Japanese have taken another approach: "What can we make that overseas markets will buy? " Consequently, Japanese trading companies have been product generalists, whereas most U.S. trading companies specialize along product lines. But the international trade environment is changing. The volume of international trade has increased and so has its complexity. Importing companies are making demands on exporters. To sell military aircraft to Switzerland, Northrop Corp. had to agree to help the Swiss market their exports worldwide. Northrop has the corporate resources to handle this demand. ("But all things considered, we would just as soon not," says a Northrop official.) What, however, would a small maker of computer peripherals do with several million dollars in tapioca it had to agree to sell in exchange for the privilege of exporting its products to Southeast Asia?

The Export Trading Company Act of 1982, signed by President Reagan last fall, removes two stumbling blocks to growth in the tiny tradingcompany industry. First, it gives bank holding companies authority to make equity investments in export trading companies. Second, it permits domestic competitors to join in an export effort without fear of prosecution under U.S. antitrust laws.

Banks, thus far, are looking very cautiously at their new privilege. And truding companies are looking very cautiously at banks. Traders are risktakers; banks are not.