The Collaborative International Dictionary of English v.0.48:

Disproportional \Dis`pro*por"tion*al\, a.
   Not having due proportion to something else; not having
   proportion or symmetry of parts; unsuitable in form, quantity
   or value; inadequate; unequal; as, a disproportional limb
   constitutes deformity in the body; the studies of youth
   should not be disproportional to their understanding.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disproportional
    adj 1: out of proportion [syn: disproportionate,
           disproportional] [ant: proportionate]