PRESIDENT REAGAN has been needling the Democrats and particularly Walter Mondale with the suggestion that they are the all-time gold medal champion big spenders. But he's a bit out of date. The Democrats are no longer the champions. The gold medal is currently held by Mr. Reagan himself.

He has spent more than the Carter administration, and more than any other administration back to World War II. He has spent more if you count defense in, and he has spent more if you take defense out. It's instructive to put the campaign speeches aside for a moment and look at the figures.

The accompanying table provides spending totals by fiscal year, as percentage of the Gross National Product. That's the best and fairest way to measure public spending. It takes inflation into account. It also takes io account the growth of the economy, as population and wealth rise. The figures here include off-budget spending, which was higher in the Carter administration than it is currently. Since Republicans sometimes argue that it's their emphasis on defense that's driving the totals up, we offer the totals both including and excluding defense. The figures through 1983 come from the president's budget last February. The figures for fiscal 1984, which ended last Sunday, come from the update published by the Congressional Budget Office in August. Spending as a proportion of GNP is currently falling because, in the present phase of the business cycle, the economy is expanding faster than the spending programs.

Mr. Reagan has sometimes suggested that spending is high because Congress keeps disobediently voting for popular social benefits. In fact, more often than not Congress has cut where Mr. Reagan indicated. Where the final spending totals for the year have been substantially larger than the original Reagan budget figures, it has usually been in areas where the administration at least shares the blame. In both 1982 and 1983, the administration's grossly mismanaged farm programs overshot their budgets spectacularly. In 1982, it turned out that the president's budget had greatly underestimated the cost of interest on the federal debt. Similarly in 1983 it underestimated the unemployment rate and consequently the cost of unemployment compensation. As for 1984, it looks as though the actual spending total will be very close to the one that Mr. Reagan originally proposed in his budget