parvus

Parvus, parva, parvum declines like bonus, but the comparison is suppletive and comes off a different root entirely: minor, minus for "smaller" and minimus for "smallest". There is no parvior. Minor is a third declension comparative with the odd neuter minus, and the corpus is full of its oblique cases, minoris, minorem, minores.

Minor natu, "less by birth", is the standard way of saying younger, and maior natu its opposite; a minor without qualification is a person under age, exactly as in English. Price goes in the ablative, so parvo emere is to buy cheap and magno vendere to sell dear. The stem left almost nothing to English on its own, but its comparative gave us minor, minus, minimum, minister, minute and diminish.