A small minority of U.S. farmers got almost 40 percent of the $6.6 billion paid out in federal crop subsidies last year, according to data compiled by the Reagan administration in a new effort to influence congressional debate on a 1985 farm bill.

The information, reaffirming a longstanding pattern of farm benefits distribution, was passed out this week on Capitol Hill, where House and Senate Agriculture committees are at sharp odds over proposals to reduce federal spending on agricultural support programs.

The administration has proposed aiming federal payments at farmers deemed to be in the greatest need, but Robert L. Thompson, assistant secretary of agriculture for economics, conceded that the plan "has been generally ignored."

The administration's proposal would reduce the current limit of $50,000 on payments to individual farmers to $20,000, and then scale it down to $10,000 over the life of the farm bill. Sen. Jesse Helms (R-N.C.), chairman of the Senate committee, has proposed cutting the limit to $25,000.

The Agriculture Department's compilations show that for six major commodities eligible for the federal subsidies, 95 percent of the 939,000 participants would fall under the $25,000 limit sought by Helms.

The breakdown also showed that 72 percent of all farms, with less than $40,000 in annual gross sales, got 22 percent of program payouts. Sixteen percent of mid-sized farms in the $40,000 to $100,000 income class got 33 percent of payments, while the 12 percent of all farms with sales above $100,000 got 45 percent of the payments.

Thompson said the new information "puts in bold perspective" the inequity of distribution of benefits to farmers through the target price system of deficiency payments intended to help them make up production costs and bolster income.

"It makes no sense to distribute money to the largest farmers," he said. "It is better to target the assistance to those who need it the most . . . . We found that the farms getting the highest payments had average equity of more than $1.1 million each."

In a departure from past practice, Agriculture Department economists added a new twist to their compilation, listing payments according to congressional districts.

"It was meant to help the members understand the problem in their own district," Thompson said. "Support of a lower payment limitation could be related to the number of farmers already bumping the cap in a given district."

That, in part, has turned out to be a political reality that has tied both the House and Senate committees in knots as they attempt to come up with new formulas that will reduce spending yet protect farmers' income in their home districts.

"It's premature to judge what will happen," Thompson said, "but I see no groundswell in the committees for cutting the payments cap."

Agriculture Department figures showed that the average dairy payment in 1984 was $22,800, the rice average was $14,300, the cotton average $8,900, corn $4,700 and wheat $4,400.

By states, Wisconsin led milk-benefits payments with $112.5 million. Kansas, with $185 million, edged out North Dakota, with $184 million, as the top wheat recipient. Iowa, with $469 million, led the corn states. Texas, at $303 million, was No. 1 in cotton payments, while Arkansas' $161 million was highest for rice, and Texas led wool payments at $17 million.

The district represented by Rep. Steven Gunderson (R-Wis.), an Agriculture Committee member, led all milk recipients, with $36.9 million last year. North Dakota, a single congressional district represented by Rep. Byron L. Dorgan (D), was tops in wheat with $183.8 million.

The top corn payment district, represented by Rep. Virginia Smith (R-Neb.), who sits on the agriculture appropriations subcommittee, got $162.8 million. The Texas districts represented by Reps. Larry Combest (R) and Charles W. Stenholm (D), both committee members, led cotton benefits with $208 million. The Arkansas district of Rep. Bill Alexander (D) led rice with $118.9 million.