Onionoo does not honor "Accept-Encoding: gzip"
Onionoo delivers large documents uncompressed, even when gzip compression is requested.
According to onionoo's documentation:
Clients should include an "Accept-Encoding: gzip" header in their requests and handle gzip-compressed responses. Only requests starting at a certain size will be compressed by the server.
Despite this, when viewing the "details" and "bandwidth" documents, they are delivered uncompressed. These are 10.5 MB and 48.6 MB, respectively.
Simply browse the /details or /bandwidth URLs @ https://onionoo.torproject.org. Use Chrome/Firefox's respective web developer tools to view relevant headers and document sizes.
These documents should be delivered using gzip compression when requested.