IN THE COURSE of explaining to Congress the small size and discreet shape of his country's defense posture, Japan's Prime Minister Suzuki spoke a line that came out this way: "We would prefer to be a wise mouse rather than a roaring lion." But is that the only choice?

The American argument with Japan over its defense role has been going on, politely, for a long time. In the main, Americans have been extraordinarily respectful of the special circumstances -- defeat in World War II and being the target of the only nuclear weapons ever dropped in anger -- that produced Japan's constitutional ban on war, its low rate of defense spending and its nuclear allergy. That these limitations are imbedded in Japanese politics is well known. The United States has been more than willing to provide an essential great-power umbrella.

The Japanese seem slow to recognize, however, that times have changed. They acknowledge freely, for instance, that they are still working on the basis of a defense plan drawn up in 1976, as though it were something sacrosanct. But one does not have to subscribe uncritically to the Reagan defense projections, which amount to a complete tearing up of the American "defense plan" of 1976, to grant that the look of things is very different in 1981. Mr. Suzuki's predecessor told Americans last year that Japan would increase defense spending by 9.7 percent (to one percent of GNP). The actual figure was 7.6 percent. One does not want to get hung up on numbers, but as indices they have their value.

In the Japanese view, their most valuable security contribution lies less in the military area than in fostering stability by helping out in Third World development. They are right to underline the link between development and security. The strain on this issue is not something new with the Reagan administration, and it is, we suspect, bound to grow.

Most of the diplomacy the two nations conduct with each other entails regulating their immense and delicate economic exchanges. These are not likely to get easier. Until now, they have been conducted in an atmosphere relatively free of differences on other issues, such as security. It is in both countries' interests that this continue. Just as there had to be earnest negotiation and compromise on the auto exports issue, however, so must there be a continuing engagement on the security issue. The Americans must listen hard, but the Japanese cannot simply explain their special conditions and sit back with folded arms. Americans do not expect Japan to become a lion that roars. But a lion that squeaks?