The problem has been fixed by Barry (copy-pasted from wp-hackers mailing list):
So, here’s the technical explanation:
It had nothing to do with the User-Agent but with the HTTP request
headers that were sent. By default CLI utilities such as wget and curl
dont send any Accept-Encoding headers. Server side we were sending a
‘Content-Encoding: gzip’ header for the tar.gz file. Since there was no
‘Accept-Encoding: gzip’ header sent by the client, an uncompressed
version was delivered to the client. It’s pretty inconvenient (as you
noticed) to have a tar.gz file that is uncompressed.
I have removed the Content-Encoding header and replaced it with the
correct Content-Type headers to identify the different download types.