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Synonyms for Proportional (same or very similar meaning)

WordNet sense 1 ((of taxes) increasing as the amount taxed increases):
graduated

WordNet sense 2 ((of taxes) adjusted so that the rate increases as the amount of income increases):
progressive

WordNet sense 3 (properly related in size or degree or other measurable characteristics; usually followed by `to'):
relative

WordNet sense 4 (being in due proportion):
proportionate


From the ODE community, based on WordNetadd/edit