To grow less
lessen, diminish, decline, abate, modify, wane, deteriorate, degenerate, dwindle, be consumed, sink, settle, lighten, slacken, ebb, lower, melt away, moderate, subside, shrink, contract, recede, shrivel up, depreciate, soften, quiet, narrow down, droop, waste, fade, fade away, run low, weaken, crumble, let up, dry up, slow down, calm down, burn away, burn down, smooth out, die away, die down, lose its edge, wither away, decay, drop off, taper off, tail off, devaluate, evaporate, fall down, fall away, fall off, slack off, wear off, wear away, wear out, wear down, wind down, rev down*, slump*; see also contract 1.
Antonyms grow*, increase, multiply.
To make less
reduce, lessen, lower, check, curb, restrain, quell, tame, mollify, dampen, compose, hush, still, palliate, sober, pacify, allay, blunt, qualify, tranquilize, curtail, subtract, render less, abridge, abbreviate, downsize, condense, shorten, minimize, diminish, slash, attenuate, dilute, retrench, shave, pare, prune, truncate, deplete, abate, mitigate, modify, make brief, digest, limit, level, deflate, compress, contract, strip, thin, bleed, make smaller, devaluate, devalue, clip, abstract, summarize, epitomize, sum up, lighten, trim, rake off, level off, take from, take off, roll back, hold down, mark down, scale down, boil down, let up, cut off, cut down, cut short, cut back, strike off, deduct, knock off*; see also compress, contract 2.
Antonyms increase*, expand, augment.