"Accept: application/vnd.npm.install-v1+json" makes responses *larger*, not smaller

[broofa]

Pretty much what the title says. Over on npmgraph.js.org I have clients making metadata requests with the Accept: application/vnd.npm.install-v1+json; q=1.0, application/json; q=0.8, */* header, as per the registry docs. However this causes the response size to increase rather than decrease due to responses not being compressed.

Is this expected?

This is reproducible in curl as follows:

With Accept: application/vnd.npm.install-v1+json header: (Notice, no content-encoding header in response)

$ curl -I -H "Accept: application/vnd.npm.install-v1+json" -H "Accept-Encoding: gzip, deflate, br" https://registry.npmjs.org/sshpk
HTTP/2 200
date: Mon, 16 Oct 2023 02:34:13 GMT
content-type: application/vnd.npm.install-v1+json
cf-ray: 816ce6cba859c76a-SEA
cf-cache-status: HIT
accept-ranges: bytes
access-control-allow-origin: *
age: 53
cache-control: public, max-age=300
etag: "6d473badd655b10d71efbc995d0f78ed"
last-modified: Sun, 26 Jun 2022 23:33:07 GMT
vary: accept-encoding, accept
server: cloudflare


$ curl -s -H "Accept: application/vnd.npm.install-v1+json" -H "Accept-Encoding: gzip, deflate, br" https://registry.npmjs.org/sshpk | wc -c
   51594

# Edit to add:  Gzipping the above content would result in 72% reduction in size(!)...
$ curl -s -H "Accept: application/vnd.npm.install-v1+json" -H "Accept-Encoding: gzip, deflate, br" https://registry.npmjs.org/sshpk | gzip -c | wc -c
   14264

With Accept: application/json header: (Notice: includes content-encoding header)

$ curl -I -H "Accept: application/json" -H "Accept-Encoding: gzip, deflate, br" https://registry.npmjs.org/sshpk
HTTP/2 200
date: Mon, 16 Oct 2023 02:34:25 GMT
content-type: application/json
cf-ray: 816ce716a85cc59c-SEA
cf-cache-status: HIT
access-control-allow-origin: *
age: 277
cache-control: public, max-age=300
etag: W/"3f4fd114960789d0e7146c756fcdecdf"
last-modified: Sun, 26 Jun 2022 23:33:07 GMT
vary: accept-encoding, accept
server: cloudflare
content-encoding: gzip


$ curl -s -H "Accept: application/json" -H "Accept-Encoding: gzip, deflate, br" https://registry.npmjs.org/sshpk | wc -c
   26373

[arcanis]

Previously reported here: #749