nginx terminates connection after 65k bytes

I've got nginx configured as a front-end to a Python application running under gunicorn, but nginx is terminating connections after about 65k of data have been sent.

For example, I've got a view which looks like this:

def debug_big_file(request):
    return HttpResponse("x" * 500000)

But when I access that URL through nginx, I only get 65283 bytes:

$ curl https://example.com/debug/big-file | wc
…
curl: (18) transfer closed with outstanding read data remaining
   0       1   65283

Note that everything works as expected when accessing gunicorn directly:

$ curl http://localhost:1234/debug/big-file | wc
…
   0       1   500000

The relevant nginx config:

location / {
    proxy_pass http://localhost:1234/;
    proxy_redirect off;
    proxy_headers_hash_bucket_size 96;
}

And nginx version 1.7.0

Some other facts:

• The number of bytes is consistent from request to request, but it varies based on the content (I first noticed it with a large PNG file, which was cut off after 65,372 bytes, not 65,283)
• 110k bytes are sent correctly (ie, "x" * 110000 returns all 110,000 bytes), but 120k bytes are not
• tcpdump suggests that nginx is sending a RST packet to gunicorn: