Underweighted rhymes with:
overweighted, equated, overweighted, weighted, abated, annotated, baited, bated, belated, berated, conjugated, dated, debated, deflated, desecrated, equated, fated, feted, gated, grated, interrelated, mated, misstated, overrated, overweighted, predated, reinstated, related, restated, sated, sedated, skated, slated, unabated, undated, underrated, unrated, unrelated, unstated, weighted
Underweighted sounds like:
unaddressed, unauthorized, undercoat, undercooked, undercut, underside, undersized, understate, understated, understood, understudy, underused, underweight, undressed, uninterested