Maybe Chomsky is considered a heavyweight because he is the pre-eminent linguistics scholar of all time whose works set the standard and provide the received view for all to attack and modify.
Actually I thinkj he understands F better than you suggest. C's human nature is pretty thin, basically a capacity to speak gramatically and produce new sentences. He also holds that there is sort of an instinct to be free, but Foucault agrees with this, as noted in a previous post, he holds that while domination is inescapable, resistance to it is inevitable. So actually they are on the same page on this point about human nature. For the rest, F likes to play up constructivist stories about human history, while C likes to talk about the evils of imperialism, so they are not singing from the same hymnbook.
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