In a classic mistmatch that one Pentagon executive said "is enough to make your eyes water," Army weaponry has gotten too big to fit into most of the Air Force planes built to carry it to battle.

"By 1986," laments Defense Secretary Harold Brown, "most Army firepower equipment will be too large for any current aircraft except the C5," Lockheed's giant transport plane. To worsen the mismatch, even the Air Force's fleet of 77 C5s could not deliver the Army's big stuff, like the M60 and XM1 tanks, to a distant place like the Persian Gulf without great difficulty and staggering expense.

At most, one C5 can carry only two of the Army's new 60-ton XM1 main battle tanks at a time. Because it costs $11,051 an hour to operate the C5, it would cost $221,010 to deliver just two XM1 tanks the 10,000 miles to the Persian Gulf.

Even though the Army is investigating lighter tanks and guns for places like the Persian Gulf, it is seven to 10 years away from transforming those ideas into fighting vehicles. In the meantime, given U.S. government worries about the Persian Gulf, the Pentagon must find a way to get heavy weaponry near likely trouble spots.

The lack of fit between Air Force planes and Army firepower helps explain why the Carter administration has tried to find nations near the Persian Gulf willing to store U.S. military equipment; why cargo ships loaded with weapons and ammunition are anchored off the Indian Ocean island of Diego Garcia, and why the administration is pressing for new transport aircraft and high-speed ships.

Pentagon leaders say they are gearing up for what they call a "one-half-half" war strategy. This calls for preparing for one big war, in Europe, for example, and small wars in Asia and the Persian Gulf.

Army leaders concede that their M60 and XM1 tanks, 155 mm self-propelled howitzer, the new 22-ton infantry vehicle and the Roland antiaircraft missile system are all too big to fit in any plane except the C5. But they said they had no other choice but to build big.

"The driving consideration was to win against the Soviet threat in Europe," insists Maj. Gen. F. K. Mahaffrey, director of the Army's requirements office.

He said that anything smaller than the M60 and XM1 tanks could not stand up against their Soviet counterparts, like the T72. But given the new focus on the Persian Gulf, the general added, the Army has just started an experiment to assess lighter tanks and guns, using the 9th Infantry Division at Fort Lewis, Wash., as the test outfit.

Another Army officer said that the world has changed since the Army weaponry now going into production was designed a decade or so ago. Back then, he argued, lightly armed Marines would be plenty to handle trouble in most of the countries around the Persian Gulf. Since then, he added, the Soviets have sent to many of those countries in the Indian Ocean region such sophisticated weaponry as T72 tanks and Mig 23 aircraft.

Civilian policy-makers, who are supposed to keep the Air Force from going in one direction and the Army in another, admit that three should have been better cordination. They blame the Air Force for spending too little on transport planes, and the Army for building too heavy. But these civilians concede it is too late to switch the course immediately.

"We obviously can't go back and start all over again on the XM1 tank," said one Pentagon civilian analyst. The first batch of those tanks has already come off the Chrysler production line. The Army has ordered 7,058 XMs, each weighing 60 tons and expected to cost $1.6 million.

The quickest fix, as Pentagon analysts see it, is to preposition more Army equipment near the Persian Gulf on both land and aboard ship; buy highspeed ships for reinforcing lightly armed troops that might have to land far from any supply base and building a giant a successor plane to the C5.

However, Pentagon plans for this successor to the C5, called the CX, have run into rough weather in Congress. The lawmakers have refused to approve the project until the Pentagon spells out just what kind of plane it wants and why.

One problem with the CX is that it is being advertised as a plane that could take Army tanks and other gear long distances and also puddle jump from one rough field to another within the battle theater. Veteran lawmakers heard the same claims for the C5, only to find out that the Air Force did not want to risk landing that plane near the fighting.

The long- and short-run requirements for the CX were put together by Brown. The Reagan administration may opt for going back to the original idea, which was to build a C5-type transport and also a smaller one for operating within a battle theater. The later plane would be the successor to the C130.

The Air Force's fleet for transporting Army firepower, besides the 77 C5s, consists of 480 C130s and 274 C141s.

Given the fact that the Army's big weaponry cannot fit into either the C130 or C141, said one Pentagon executive, "we should have built them bigger. It would not have cost that much more."