Really? There are plenty of valid reasons why XMPP is not suitable for mobile usage (most notably, the proprietary push notification systems that all platforms insist you have to use), but XML isn't one of them. If you use TLS with XMPP (as gtalk does [1]), you get the compression protocol for free, but even if you can't, there's XEP-0138 [2], which specifies the use of zlib and has been in development for over 12 years now, and finalized since 2009. XML is fugly, but it works. At the scale Google operates, it is laughable that the use of XML in chat protocols could possibly impact their bandwidth accounting.
[1] https://developers.google.com/talk/open_communications#devel...
[2] http://xmpp.org/extensions/xep-0138.html