President Reagan today renewed a frequent complaint of recent weeks that his defense buildup has been whittled down to even less than what President Jimmy Carter projected would be spent this year.

Reagan's lament in his weekly Saturday radio address came after he accepted another slowdown in his military expansion plans last week as part of a deficit compromise with Senate Republican leaders.

But even with this new compromise, Reagan already has succeeded in launching a Pentagon buildup that far exceeds anything Carter envisioned. He has set in motion a weapons assembly line for which the bills are coming due in a "bow wave" of spending that may be difficult to slow.

For all the debate over defense budget "cuts," Congress has, in fact, not seriously restrained Reagan's military expansion. Rather, it has given him 97 percent of what he originally asked for between 1981 and 1985, when adjusted for inflation.

"The truth is, Congress is opposed to the big defense number, but not the main elements of the buildup program which is now in its fifth year," Office of Management and Budget Director David A. Stockman has said.

"And that is the rub: big savings come from canceling programs and reducing force structure, not from paper growth-rate exercises" in which lawmakers "apparently plug in their gym locker combination -- 0-3-3 -- to get an answer," he said in a March 28 speech to the National Press Club.

Stockman, who has waged his internal battles to restrain the Pentagon buildup, pointed out that Congress voted $100 billion over the last five years to modernize U.S. conventional aircraft. For the next five years, the administration has budgeted about $130 million to finish producing what already has been started.

"Despite its aversion to big numbers," he said, Congress has voted for these aircraft "overwhelmingly" time and again "for compelling reasons of military capability," and is likely to continue doing so.

The story is similar in the case of the bomber and sea-based legs of the strategic "triad" for which Stockman said Congress approved $50 billion in the last five years and the administration has budgeted another $40 billion through 1990 to complete production of the B1 and five additional Trident submarines and for early production of the Trident II missile.

Likewise, Congress "enthusiastically ratified the 600-ship Navy objective" and approved $57 billion the last five years to buy 100 ships; another $60 billion is budgeted for the next five years to complete the program, he said.

Congress also approved a small increase in force structure, bringing total uniformed strength to 2.1 million, the budget chief said. Military compensation cost $350 billion the last five years and only "slightly more" is set for the next five.

Scrapping some of these ships, planes, missiles and personnel increases could produce the kind of major deficit savings that many in Congress have been seeking, but lawmakers have been unwilling to do that throughout the first Reagan term.

Instead, the defense budget has been squeezed in other relatively painless ways. For example, the buildup was launched at a time of high inflation. As inflation has subsided, fuel and other materials can be bought for less.

These inflation "savings" provided a cushion, allowing Reagan to reduce his defense budget requests without crimping big-ticket weapons systems.

A second technique has been to stretch production of weapons over a few more years rather than cancel them. For example, in 1983, the administration estimated that it would buy 830 M2 Bradley armored troop vehicles in 1985; that has been reduced to 655. The rest will be stretched out to future years.

Reagan's latest defense compromise appears to follow this pattern. He agreed to accept a 3 percent increase in spending authority above inflation instead of the 5.9 percent he requested in February. The Senate Armed Services Committee, under Chairman Barry Goldwater (R-Ariz.), approved this trim in Reagan's request without eliminating any major weapons systems, although the number of MX missiles to be deployed was pared.

"There are things that I think were worthwhile that will not be done now for a while -- be delayed," Reagan said Friday as he left for a vacation here. "But it will be an increase, continuing increase, and no weapons system will be slowed down or cut out of the military budget so we can honestly say that with this our national security capability has not been reduced."

This is Reagan's characteristic approach. Although he insists that defense spending must increase to protect the nation's security, he also seizes a compromise and declares that it will not hurt national security.

Still, there are indications that Congress faces intensifying pressure to crimp military hardware and personnel in the search for deficit savings. Reports of costly spare parts and weapons that do not work have come just as polls show the American people growing more skeptical about the need for a continued Pentagon buildup.

Last week, the Senate Armed Services panel also developed, but did not recommend, a "zero growth" above inflation defense bill that froze military personnel levels for next year and eliminated the Army's problem-plagued Divad battlefield antiaircraft gun. House Armed Services Committee Chairman Les Aspin (D-Wis.) predicted again last week that the House would vote for zero growth over inflation.

Critics say Congress is unwilling to scrap major weapons systems in part because of the economic punch -- jobs -- they provide for their districts.

"Systems are spread everywhere," said Gordon Adams, director of the Defense Budget Project, a research group that has been critical of Reagan defense spending plans.

"The MX was over 20,000 jobs in five or six states. They are important states with important representatives -- California, New York, Massachussetts. A member of Congress is elected to defend his district and its interests; they are hostages to the situation. Do you expect them to vote against jobs? It is an extremely unlikely form of political behavior to expect them to do that."

Without wholesale scrapping of military hardware, or deep cuts in personnel, military spending is picking up momentum from past commitments.

This is because weapons programs take many years to complete. The spending for them lags far behind the vote by Congress to authorize the project.

Reagan asked Congress for $313.7 billion in new spending authority for fiscal 1986. But only about half of that money -- 54.6 percent -- would be spent in the first year.

Out of every dollar of Pentagon spending next year, 38 cents can be traced to obligations made in previous years. Another 43 cents is for payroll and related costs. Only 12 cents is linked to new budget authority voted by Congress.

This contributes to the bow wave effect in which the bills come due later for weapons authorized in earlier years. Some analysts fear that the buildup has been underpriced and predict there will be even greater pressures for spending later to operate and maintain the new weapons adequately.

Reagan is "on the brink of locking into place an irreversible 'weapons-driven' defense budget," according to Adams' research group.

The share of defense spending devoted to "investment," which is weapons procurement, research and development, and military construction, has grown in the Reagan years from 37.7 percent to 50 percent in next year's budget request, the group reported.

At the same time, the share of the budget going to operations, which includes personnel and operations and maintenance, has declined from 60.8 percent to 48.4 percent during the same period.

Although Reagan complains that Congress has reduced his defense budget to below the levels envisioned by Carter, he has achieved far more, after adjusting for inflation, than Carter ever projected.

Carter's 1981 projections were made at a time of high inflation; then Reagan and Congress layered on even higher budgets.

When inflation dropped, the result was that Reagan reaped more purchasing power for the same defense dollar.

According to Alice Maroni of the Congressional Research Service, when inflation is taken into account, Reagan improved on Carter's last projection by $124.5 billion in spending authority, or 10.9 percent, between 1981 and 1985.

In other words, Reagan put into place a hefty supplement to Carter's defense plans, and did it despite the concessions he made to Congress along the way.