The Collaborative International Dictionary of English v.0.48:

Undersized \Un"der*sized`\, a.
   Of a size less than is common.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undersized
    adj 1: smaller than normal for its kind [syn: undersize,
           undersized]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "undersized":
   Lilliputian, Tom Thumb, bantam, bony, dumpy, dwarf, dwarfed,
   dwarfish, elfin, flat, flat-chested, fleshless, gangling, gangly,
   gaunt, gawky, incipient, lank, lanky, lean, lean-fleshed,
   lean-looking, little, meager, midget, nanoid, petite, pygmy,
   rawboned, rudimental, rudimentary, runty, scraggy, scrawny,
   scrubby, short, shriveled, shrunk, shrunken, skeletal, skinny,
   slight, small, spare, spidery, spindling, spindly, squat, stunted,
   thin-bellied, thin-fleshed, tiny, twiggy, underdeveloped,
   undersize, underweight, undeveloped, wizened