Caspar Willard Weinberger will be something of a contradiction when he reports to duty as Ronald Reagan's secretary of defense. He will be the onetime budget-cutter coming to preside over the biggest military spending increase in the nation's peacetime history.

"He'll handle it," predicted one of his admittedly admiring aides from Weinberger's days at the White House. "After all, the Pentagon is just a military HEW," a reference to the sprawling Department of Health, Education and Welfare which Weinberger ran from 1973 to 1975.

As the 41st secretary of defense, Weinberger will be the personnel manager for 1 million civilians who work for the Pentagon and 2 million men and women in uniform. How well he grapples with their problems will help decide whether the United States sticks with the all-volunteer military or returns to the draft.

More than half the $200 billion Pentagon budget that Weinberger will inherit for fiscal 1982 will go toward paying, feeding and housing people. Much of the rest is for costly, complex weaponry, on which Weinberger will have to make decisions without benefit of the technical knowledge his predecessor, nuclear physicist Harold Brown, has brought to the job.

The 63-year-old Weinberger is a lawyer-manager, not a technocrat. "He would have been the perfect choice for attorney general," one of his former aides said yesterday. "He doesn't know anything about defense," complained Rep. Samuel S. Stratton (D-N.Y.), a senior member of the House Armed Services Committee. "Oh, oh. Here comes Cap the Knife," said one concerned colonel in using the nickname Weinberger picked up during his days of forcing economies as director of the White House budget office from 1970 to 1972.

"Remember," pleaded one Weinberger former deputy, "Cap is a surgeon, not a butcher. Then again, those generals and admirals over there want somebody with a shovel."

Shoveling money into military programs is something Weinberger will not be doing as defense secretary, his friends assured callers. Instead, they predicted he will employ his traditional lawyer-manager approach of cross-examining the people who are supposed to know the most about the big issues and then make decisions with refreshing speed for a federal bureaucrat.

The record suggests Weinberger has the smarts to offset whatever he may lack in military expertise or political cunning: Phi Beta Kappa at Harvard, graduating magna cum laude in 1938; San Franciso lawyer; California legislature, 1952-58; California finance director for then-Gov. Reagan, 1968-69; chairman of the Federal Trade Commission for President Nixon, 1970; deputy director and then director of the Office of Management and Budget, 1970-73; counselor to Nixon, 1973; secretary of HEW, 1973-75; general counsel and vice president of the Bechtel Corp., 1975 until now.

His military service consists of four years in the Army. He entered as an infantry private in 1941 and was discharged as a captain in 1945 after serving on Gen. Douglas MacArthur's intelligence staff.

"You can't overstate how close Reagan and Weinberger are," said one Republican leader who had a hand in selecting the Cabinet. Reagan has called Weinberger "my Disraeli."