Each summer since President Reagan came to Washington, there has been a tremendous struggle over federal taxes and spending. Now that the second summer's work has been signed into law, it's possible to take stock of the profound changes overtaking the basic pattern of the budget.

The comparison begins with the fiscal year 1980, the last full year before Reagan took control. The following numbers for fiscal 1983, the year that starts on Oct. 1, come from the Congressional Budget Office's most recent revision of its Economic and Budget Outlook, published this month.

In spending policy, the changes are the ones that you would expect. But the magnitude of those changes is dramatic. Here are a few numbers, representing percentages of total spending (excluding interest on the national debt) by several major categories. [TABLE OMITTED]

Interest is not included in this definition of spending because it is set by the commercial interest rates rather than by policy, and including it would obscure the view of the purposeful decisions that the president and Congress are making.

These figures reflect, not the budget that Reagan originally proposed in February, but the budget as Congress has revised it over the past seven months. Under the original Reagan budget, the allocation to defense would be somewhat higher.

Nearly two-thirds of income security spending is Social Security. Most of the rest is other pensions, and unemployment compensation. Food stamps and several other welfare programs, small by comparison, are also in there. Similarly, about two-thirds of the spending on health goes to Medicare for the elderly, and most of the rest is Medicaid for the poor.

It's worth noting that about half of federal spending is now pensions plus various smaller income support programs and medical benefits attached to them. These categories, with defense, were three- fourths of the budget as the Carter presidency ended. Now they are five-sixths of it.

The two tax bills of the past two summers have shifted the balance among the various sources. The numbers below represent percentages of federal revenues. [TABLE OMITTED]

Despite two large cuts in income tax rates, and a third coming next July, the share of federal revenues raised by the personal income tax will be almost exactly the same next year as it was in the Carter years. The striking drop has come in corporate taxes. It is being offset chiefly by the rise in social insurance taxes. Most of that is the Social Security payroll tax.

Politicians and economists are currently discussing the flat income tax and whether the country should move toward it. That little table above is a reminder that the country is, in fact, moving toward it rapidly. The personal income tax is at present progressive -- meaning that people with higher incomes pay larger shares of their income than the poor. But the Social Security tax is regressive. It's a flat tax on earnings up to $32,400 a year, with no exemptions and no deductions.

Taken together, the personal income tax and the Social Security tax form a structure that is still probably mildly progressive -- but only very mildly.

So far we've been talking about the changing allocations within the budget. That leads to another crucial question: how big is the budget under Reagan's guidance?

The best way to judge it is in relation to the size of the whole economy. The numbers below express total spending -- this time including interest payments -- and tax revenues as percentages of the gross national product. [TABLE OMITTED]

Far from declining, the first Reagan budget in 1982 rose sharply in relation to the size of the economy. It wasn't wholly due to the recession. It resulted, in roughly equal importance, from rising defense spending, rising interest costs and rising spending on income security and health. In 1983, according to the CBO estimates I am using here, spending will finally begin to turn downward as a percentage of GNP -- although slowly.

In contrast, the Reagan policy on taxes took effect immediately and sharply. Federal revenues, as a share of GNP, were rising when Reagan took office. But, following the 1981 tax cut, they began dropping, and, as the table shows, the drop accelerates rapidly as the government goes into the coming year.

Those figures for 1983, by the way, assume moderately good economic growth next year. They are based on the CBO's forecast of recovery from the recession with no increase in inflation.

If those numbers for 1983 turn out to be correct, and revenues are 19 percent of GNP, Reagan will have lowered the tax burden to the level of the middle 1970s -- the Ford era. It will still be larger in proportion to the whole economy than it was in the Nixon period, or in the early 1960s before Vietnam.

As for spending, at 23.7 percent of GNP it will be lower than this year but higher than any other year since World War II, and a good deal higher than in the Carter years. At 4.7 percent of GNP, the deficit will also be higher than any since World War II.

So far, Reagan and Congress have done most of the cutting in the small spending programs that are weakly defended, like welfare and food stamps, rather than in the big ones like Social Security that serve very large numbers of people. To close the deficit by cutting spending, as Reagan keeps promising, is impossible without heavy reductions in at least two of the three largest groups of programs -- defense, health care or the pensions. If that seems unlikely to you, there are only two possibilities. Either the deficit stays at the present very high levels, or there must be further tax increases.