Antonyms of slim:
gain, put on, large, fat, big, chunky, squat, stocky, stumpy, thick, thickset, bulky, massive, voluminous, thickish, widish, broad, wide, beefy, fleshy, heavy, heavyset, stout, weighty, brawny, burly, husky, dumpy, pudgy, roly-poly, stubby, paunchy, potbellied, flabby, soft, buxom, zaftig, fleshed-out, full, hippy, round, chubby, corpulent, gross, obese, overweight, plump, portly, rotund, tubby, adequate, enough, satisfactory, sufficient, tolerable, fecund, fertile, fruitful, prolific, rich, lavish, luxuriant, blooming, bursting, flourishing, proliferative, swarming, teeming, thriving, excess, extra, surplus, abundant, ample, bountiful, copious, generous, liberal, plenteous, plentiful, great, distinct, significant, considerable, goodly, healthy, largish, respectable, sizable, substantial, tidy, good.
Examples of usage:
  1. We are often applied to for cauliflower plants as late as May, but the chances of their forming heads when planted in May are slim indeed.
    "The Cauliflower", A. A. Crozier.
  2. He drew out the bill and handed it to Slim for his inspection.
    "The Girl Aviators' Sky Cruise", Margaret Burnham.
  3. Slim, sweet and graceful, she looked up into his somber face.
    "The Sherrods", George Barr McCutcheon.