Christmas came early for Defense Secretary Caspar W. Weinberger when President Reagan played Santa Claus last week and generously approved the overstuffed budget that the Pentagon has come to expect from him.

Defense spending increases averaged 9 percent annually after inflation during Reagan's first term, more than fulfilling his campaign promises of a military buildup. Ignoring all the signs of a congressional storm over the growing federal deficit, Weinberger is determined to keep it soaring.

Budgets are boring but instructive. Defense-spending authority exceeded $284 billion in fiscal 1985 on an upward track that under Reagan's budget will top $313 billion in 1986 and reach $401 billion in 1988.

Against this backdrop, Weinberger's supposed $8.7 billion in defense budget "reductions" for fiscal 1986 would look pretty paltry even if they were real. In fact, some of the cuts are suspect. The reductions include nearly $1 billion in paper savings from a recomputation of fuel costs and inflation, achieved by the Pentagon equivalent of Office of Management and Budget Director David A. Stockman's magic pencil.

The reductions also include $2.5 billion of supposed program cuts that will not affect a single military program. That money, as far as anyone can tell, will be drawn from $4 billion in discretionary funds that Weinberger had not allocated to any of the services.

This dubious defense budget is the product of a month's skirmishing that is informative about the workings of the Reagan administration and some of its principal players. Weinberger has emerged again as the most skilled bureaucrat in the crowd, in the best and worst senses of the word.

When Weinberger was at what is now called the Department of Health and Human Services, he was an effective advocate of social programs. When he was budget director, he was "Cap the Knife." As defense secretary, he is the skilled protector of Pentagon interests who quotes the president to win him over, as when he contends that defense spending must be determined by the Pentagon perception of the Soviet military budget.

But Weinberger also is beset by a bureaucrat's parochialism. He finds it difficult to put aside the needs of his agency for a larger purpose. The arguments of Republican stalwarts such as Sen. Paul Laxalt (Nev.), who contends that national security requires a strong economy as well as a strong defense, do not penetrate the bureaucratic mind-set. Weinberger does not realize that it is no longer 1981 on Capitol Hill.

The December budget battle also reinforced some basic perceptions about Stockman and White House chief of staff James A. Baker III. It showed that Stockman, who this time submitted an honest and detailed budget proposal, is still considered too clever by half, both in the Oval Office and on Capitol Hill. And it showed that the cautious Baker, despite his political and negotiating skills, is no match for Weinberger on his own turf.

What the process tells us about Reagan is most interesting of all. The president deliberately has kept out of public view, and those who have dealt with the Great Delegator have found him cheerfully unaffected by the portents of the growing deficit.

As always, Reagan leaves the details and the dirty work to others. As always, he is difficult to stampede on what he considers a matter of principle. He has not acknowledged that the domestic spending cuts he seeks and his pledge to hold the line on taxes both are jeopardized by the defense budget.

But it is a mark of Reagan's enduring political strength and style of governance that he is rarely damaged in the public mind by differences in his official family or the instransigence of his Cabinet members. There are many theories about why this is so. Mine is that Reagan seems so distanced from the events of his administration and so essentially nonpolitical that people are apt to blame his surrogates when something goes wrong.

Reagan's proclivity for delegation is frustrating to aides and senior Republican members of Congress, some of whom say the president has abdicated his leadership role. But it leaves him free to rush in and rescue the girl he has tied to the tracks, to play the hero after Congress moves in to save him from himself. This has happened before and could happen again, especially since many of his congressional loyalists view the proposed defense budget as an invitation to disaster.

It was Weinberger, not Reagan, who was front and center before the cameras last week defending the gift budget he had asked for and received. When Christmas is only a memory, that may turn out to be most fortunate for the president. Reaganism of the Week:

Thursday an aide cautioned Reagan against taking his new puppy, Lucky, into the Oval Office for fear that the sheep dog might urinate on him there. "Well, why not, everybody else does," Reagan replied.