THE MOST substantial cuts in federal spending next year will fall on the defense budget, and those cuts were imposed by Congress. Rudolph G. Penner, the director of the Congressional Budget Office, makes the point that there is more muscle than appears at first glance in the budget resolution that Congress passed at the beginning of this month before it went on holiday. Mr. Penner is not entirely neutral in these matters, since he works for the two houses of Congress, but he is nonpartisan and, more to the point, he is good at arithmetic. Defense spending is now 6.5 percent of the gross national product, and the congressional reductions will hold it there rather than allow it to keep rising as Mr. Reagan wanted. That's a change of first importance.

As Mr. Penner concedes, the budget resolution didn't go as far or as fast as a great many close watchers had hoped -- and they have a fair point. But he argues that the resolution nevertheless deserves attention. The federal budget is now on a track that, without further legislation, would produce steadily rising deficits. President Reagan proposed changes last February that would have kept the deficits in their present range over the next several years -- at least if the economy grew rapidly. The congressional budget resolution points it on a downward path as long as the economy keeps growing. And if there's a recession? At least the consequences will be much less dangerous than they would have been under the president's February budget, Mr. Penner argues -- and he's right.

In retrospect the crucial point in this year's struggle over the budget was early last April, when the Senate Republicans succeeded in imposing their defense numbers on the compromise that they negotiated with Mr. Reagan. There was subsenquently a lot of quarreling over other parts of it, but the administration chose (wisely) not to make a further issue of defense. The additional cuts in non- defense spending that the administration evidently intends to press this autumn would amount, at most, to one-tenth the amount that Congress has already taken out of defense.

It was a remarkable achievement. The president, just reelected in one of the great sweeps of American political history, wanted to continue the rapid increases in defense spending. Congress, which had been paying close attention to the ways in which the money was being spent, said: no, it's time to level off. And they made it stick. The next time you hear Mr. Reagan denounce the spendthrift Congress, you might keep this episode in mind.