PRESSING THE FLESH. Some Washington heavyweights are shedding excess poundage. White House press secretary Marlin Fitzwater has lost so much weight (55 lbs.) on an Optifast liquid diet that even Mikhail Gorbachev commented on it at Malta. House Speaker Tom Foley, who has dropped 20 lbs., vows to get rid of 25 more. House Ways and Means chairman Dan Rostenkowski, who balloons every winter from holiday partying, is living mostly on grapefruit. Republican national chairman Lee Atwater's Slim-Fast diet of milkshakes was emulated by some of his dutiful staff, but the boss has let them down. Explains an aide: "Lee's idea of a diet is to drink two Slim-Fast meals, then go out and eat 25 chicken wings dipped in honey for dinner." His weight loss so far: 0 lbs.