Americans are cutting back on their eating but still aren't winning the battle of the bulge: their weight remains about the same on fewer calories. Agriculture Department figures show that men are consuming 10 to 15 percent fewer calories, depending on age, and women are consuming 5 to 10 percent fewer than in 1965-66. The most dramatic decline was for infants: 34 percent. But "mean weights of people in most groups in 1977 were similar to mean weights in 1965," said Eleanor Pao of the Consumer and Food Economics Institute. "Thus the drop in caloric intake does not appear to be associated with loss of weight." Since the figures are in calories, regardless of form, it doesn't appear that junk food or dietary changes are to blame. Pao said she feels the explanation is "a sedentary lifestyle . . . more common in the 1970s." The data also yielded some other tidbits: Caloric intake for all ages and sexes was 10 to 25 percent below the recommended daily allowance for 1974. Average protein consumption remained the same or decreased in all age/sex levels. Infants showed the largest decline, 40 percent. Fat intake also decline for all groups. Vitamin C intake increased sharply.