Despite all the talk on Capitol Hill about reducing deficits by putting the Pentagon on a diet, the weight-loss charts are not encouraging.

The House and the Budget Committee of the Senate have voted to cut the Reagan defense buildup at least by half. The drafters of these cuts concede that they will not save much money right away, but envision big savings in the future. But others doubt that sizable savings are in store unless Congress and the Pentagon mend the ways they do business.

Only two years after bowing to President Reagan's demands for a huge military buildup, Congress clearly is having second thoughts about the cost. And there is a strong undercurrent of skepticism over the numbers game that Congress is playing as it debates how much to cut Reagan's request for a defense spending increase of 10 percent next year after inflation.

The House wants to limit this "real" increase to 4 percent. The Senate Budget Committee settled on 5 percent. The administration has signaled that it might go along with 7.5 percent on the Senate floor under pressure from Senate Republican leaders, who want to ensure a House-Senate compromise in the 5-to-6 percent range.

But skeptics in both houses doubt whether these percentages will matter much as long as Congress fails to force changes in the way the Pentagon spends its buildup money and accounts for the costs. And they are not optimistic that Congress will do so, since many members have a pork-barrel interest in various military projects.

In an otherwise pessimistic assessment of budget prospects in Congress, Office of Management and Budget Director David A. Stockman observed recently that, even under the worst circumstances for the administration, a real growth rate of 5 to 6 percent could be achieved "through effective management of congressional constituencies for major weapons systems and military operations."

Even its sharpest critics marvel at the way the Pentagon has locked in its previous gains in a variety of ways, including what they describe as "low-balling" cost estimates and scattering down payments on weapons systems that then become difficult if not impossible to cancel.

The concern over real savings from defense comes from lawmakers as far apart on the political spectrum as Sen. Charles E. Grassley (R-Iowa) and Rep. Les Aspin (D-Wis.).

"Unless there's dramatic change in the way the Pentagon does business, there's not going to be savings worth talking about," said Grassley, a Senate Budget Committee member who has joined a growing number of conservatives in complaining about throwing money at defense as well as domestic social welfare programs.

"Oh, sure, you can get the savings, but you'll probably get it the wrong way, by cutting things like pay and readiness," said Aspin, a more liberal critic who is chairman of the House Armed Services personnel subcommittee as well as a senior member of the House Budget Committee. "People talk in campaigns about waste first and then big weapons systems . . . but Congress doesn't cut big weapons sytems and it probably adds more waste than it cuts."

In what has become known among cynics as the "gotcha" speech, Navy Secretary John F. Lehman Jr. all but confirmed the critics' suspicions that the military already may have won the war over spending. As quoted in the National Journal, Lehman told a Brookings Institution seminar recently that it may be "too late" to halt the drive for a 600-ship Navy. "We've already accomplished it because we front-loaded the budget," Lehman was quoted as saying.

For next year, when deficits will be pushing $200 billion for the second year in a row, none of the budget plans that are given a serious chance in Congress would do more than nibble at the edges of Reagan's proposed defense spending level for fiscal 1984, amounting to roughly $240 billion.

Even the the budget adopted by the Democratic-controlled House, which Reagan once suggested would bring joy to the Kremlin, would save less than $10 billion.

As Stockman noted, much of the spending the administration has proposed for defense, as opposed to authority for future contract commitments, is "built-in for pay, operations and procurement programs already under way" and can probably not be touched regardless of "congressional rhetoric about deep cuts."

For future years, the projected savings begin to swell, yielding cumulative five-year deficit reductions of $164 billion under the House plan and $89 billion under the Senate committee plan.

Even these savings pale by comparison with overall spending contemplated for defense through 1988: figures calculated by the Senate Budget Committee at roughly $1.5 trillion under its plan, $1.6 trillion under Reagan's and $1.4 trillion under the House version.

Moreover, the projected savings are not cast in concrete. What worries many lawmakers is that they will melt as the costs of multi-year procurement programs soar beyond initial projections and the Pentagon needs more money for maintenance, operations and personnel.

In the first two years of his buildup, Reagan has added considerably more to Carter administration projections for procurement than he has added for maintenance and operations. This leads some lawmakers to fear a "readiness" gap that will lead in the future either to soaring costs or a kind of muscle-bound weakness in the country's defense posture, or possibly both.