For the past six years, the big news about large corporations has been how little they pay in U.S. taxes.

Rep. Charles A. Vanik (D-Ohio), a member of the House Ways and Means Committee, regularly issues studies implying that many big American firms escape payment of taxes entirely despite huge profits that often run into billions of dollars.

The latest such flush of statistics, which Vanik made public two weeks ago, shows an apparent worsening of this trend: Seventeen major American firms that together earned $2.6 billion in 1976 paid no U.S. income taxes that year, Vanik asserts - up from 11 that escaped taxes in 1975.

Now comes a pair of Treasury studies that dispute two of the pet contentions long fostered by Vanik and some liberal "reformers" - first, that sizeable numbers of big firms avoid payment of taxes altogether, and second, that giant corporations enjoy a lighter tax burden than so-called small businesses.

The surveys, taken from actual tax returns, and not simply from the firms' annual reports, on which the Vanik studies rely, show that the nation's biggest firms - those with assets of $1 billion or more - pay taxes that on average amount to between 36.9 and 43.8 percent of their income, a respectable proportion by any standard.

They also show that the larger the corporation, the highter its effective tax burden is likely to be. So, for example, while the largest corporations pay a 39.9 to 43.8 percent rate, companies with assets of $50,000 or less pay only 17.9 percent on average - or half the big corporations' rate.

Moreover, the same trend holds no matter how the company's tax burden is calculated - on the basis of the percentage of its U.S. income that the firm pays in federal taxes here, or the proportion of its worldwide income that it pays in taxes to foreign governments as well.

That the Treasury's results differ so markedly from the picture Vanik paints each year is explained by a second Treasury study that points out the flaws in the Vanik analysis - mainly that he mixes apples and oranges by comparing the firms' U.S. tax liability with the income they earn worldwide.

The federal government understandably give multinational companies credit on their U.S. tax bills for the taxes they pay to foreign governments - to avoid taxing them doubly on income they earn abroad. Vanik's method of comparsion ignores these realities. (The firms may use the credit only to offset U.S. taxes on foreign income, not on money they earn at home.)

The tax laws also allow companies to offset their taxes through legitimate domestic tax breaks, such as the tax credit given business for spending on new equipment. Neither Vanik nor most other critics has proposed eliminating these credits. So why badger companies for using them? No one hits a homeowner for taking advantage of his mortgage interest deduction.

Finally, Vanik's yearly reports allege that the companies he cites have paid no U.S. income taxes, when actually he means they had no "net" tax liability. The difference is a small one, but important in maintaining the credibility of his study.

Say that a firm accrues $10 million in tax credits of one sort or another, but because of limitations in the law it can only use $8 million to offset its U.S. taxes. Technically, it has no net U.S. tax liability. But it still may be sending the Internal Revenue Service a fat check for U.S. income taxes owed.

What keeps Vanik's annual surveys in the headlines is that they do demonstrate validly one important point: That the tax breaks and credits Congress has enacted have grown so large and numerous that a good many companies now can use them to reduce or eliminate their net tax liability.