Yes, that's quite probably it!
Then I guess it's on Jerome to weigh in, I don't know the exact reasoning for doing so, I would have thought it would be more appropriate to hash the request body in the cache key, but maybe that's not possibly using nginx?
OpenCPU allows for request bodies up to 500M by default, what kind of settings would be necessary to make that (or something more reasonable like 50M) play with the buffer? I'm guessing the buffer doesn't have to be as large as the maximal request, right?