proportional, proportionate

Both words mean 'in due proportion, comparable', and are used almost interchangeably, except in certain fixed expressions such as proportional parts and proportional representation. Otherwise, when proportional occurs it tends to be in more formal and technical contexts:

By 1979 all the beds and hospital places, which were provided up to the prescribed national levels proportional to the catchment population, had been opened—D. Tomlinson, 1991

The United States does not have the vote that is proportionate to its funding—CNN news transcripts, 2004 [Old English (up to 1150)C].

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