Are slimmer coaches better coaches? The weight of the evidence says yes. The coaches of six NFL teams — the N.Y.Giants, San Diego Chargers, Seattle Seahawks, L.A. Raiders, Philadelphia Eagles, and Washington Redskins — spent mealtimes this season sucking down ”thick, delicious” Ultra Slim-fast shakes. The result: a combined loss of 249 pounds (about the weight of Giant tight end Mark Bavaro) and a combined gain of seven victories over 1989’s performance. At season’s end, four of the six had made it into the play-offs, and only the Eagles had the worse record than the year before. Each coach received a six-figure fee and a new wardrobe for appearing in the company’s ads. Weigh-ins twice a month helped keep the brigade svelte. Philadelphia’s Buddy Ryan (minus 34 lbs.) maintained his game frame with an unorthodox version of the diet. ”I mix (the shakes) with a banana and a diet coke,” he says. Perhaps he should have been stricter. The Redskins, guided by Joe Gibbs (minus 38 lbs.), beat Ryan’s Eagles in the first round of the play-offs by a score of 20-6.