WordNet sense 1 (a process of becoming smaller or shorter):

decrement

WordNet sense 2 (a change downward):

drop-off, lessening

WordNet sense 3 (make smaller):

lessen, minify

WordNet sense 4 (the act of decreasing or reducing something):

diminution, reduction, step-down

WordNet sense 5 (decrease in size, extent, or range):

diminish, fall