The expected content length is only set when the server provides it, such as by a Content-Length response header. A -1 size means the expected content size is unknown.
If you set Accept-Encoding: gzip on your request, the URL loading system will fib and tell you the expected size is -1, no matter what Content-Length the server sends. This is because it decompresses the data before passing it to you, but it can’t know the final uncompressed size till all the data has been downloaded, which is well after you receive this callback.