As far as I can tell PyPI supports returning 304 Not Modified if the response it would send matches the etag in the If-None-Match request header. If you’re polling PyPI periodically that’ll save duplicate downloads.
Unfortunately, the current implementation just reuses the already-existing json_release function, so I don’t know how much reduction in payload size it will actually provide.
A modified implementation (e.g., one simply omitting the releases key in the payload, which is where I guess most of the bloat comes from), would be easy enough to craft, if this change were desired.