Undersized rhymes with:
exercised, oversized, unexercised, anglicized, capsized, circumcised, criticized, downsized, emphasized, excised, exercised, fantasized, full-sized, hypothesized, metastasized, midsized, ostracized, outsized, overemphasized, oversized, politicized, publicized, romanticized, synthesized, unexercised
Undersized sounds like:
unaddressed, unauthorized, undercoat, undercooked, undercut, underside, understate, understated, understood, understudy, underused, underweight, underweighted, undressed, uninterested