When stream=True iter_content(chunk_size=None) reads the input as a single big chunk

According to the documentation when stream=True iter_content(chunk_size=None) "will read data as it arrives in whatever size the chunks are received", But it actually collects all input into a single big bytes object consuming large amounts of memory and entirely defeating the purpose of iter_content().

Expected Result

iter_content(chunk_size=None) yields "data as it arrives in whatever size the chunks are received".

Actual Result

A single big chunk

Reproduction Steps

from requests import get
URL = 'https://dl.fedoraproject.org/pub/alt/iot/32/IoT/x86_64/images/Fedora-IoT-32-20200603.0.x86_64.raw.xz'
r = get(URL, stream=True)
for b in r.iter_content(chunk_size=None):
    print(len(b))

prints

533830860

System Information

$ python -m requests.help

{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": "2.9.2"
  },
  "idna": {
    "version": "2.9"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.7.6"
  },
  "platform": {
    "release": "4.19.104+",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "1010107f",
    "version": "19.1.0"
  },
  "requests": {
    "version": "2.23.0"
  },
  "system_ssl": {
    "version": "1010107f"
  },
  "urllib3": {
    "version": "1.25.9"
  },
  "using_pyopenssl": true
}

Start with Response.iter_content and the linked API documentation, then run the provided Python reproduction against the Binder example to observe chunk sizes and memory behavior. Done means iter_content(chunk_size=None) yields data as it arrives rather than collecting the entire response into one bytes object.