Proportional, proportionate, commensurate, commensurable are often used without marked distinction because all mean being duly proportioned to something else.
Proportional and proportionate both imply due proportions either to a related thing or things, or of things that are related (as by belonging to the same set, series, design, or construction, or by being the effect of a cause or the response to a stimulus).
Proportional is the more usual term when a constant and often mathematically precise ratio between corresponding aspects (as size, amount, number, or length) of related things is under consideration; thus, a proportional tax is one assessed as a constant percentage of the value (as of income or realty) being taxed; a proportional wage is a fixed percentage (as of gross sales or profits).
Proportional may be used, but proportionate is more often used, when the term is intended to imply the adjustment and sometimes the deliberate adjustment of one thing that bears a reciprocal relationship to another thing, so that both are in keeping with each other or not out of keeping with what is just, fair, due, or reasonable.
Commensurate and commensurable differ from the preceding words chiefly in carrying a stronger implication of equality between related things each of which has a value (as of measure, degree, or intensity) that is intimately related to that of the other.
Sometimes both terms, but especially commensurable, differ from the other words in implying a common scale of values by which outwardly different things can be shown to be equal or proportionate in some significant way.