I have trouble believing the GPX file is delaying the implementation of a "nano" cache size. The current GPX file already allows for the easy addition of new cache sizes with the existing "container" tag (e.g., <Groundspeak:container>Nano</Groundspeak:container>). The same applies to new cache types (e.g., <Groundspeak:type>Tomb Raider</Groundspeak:type>), new log types (e.g., <Groundspeak:type>Warning</Groundspeak:type>), and attributes (<Groundspeak:attribute id="99" inc="1">Challenge</Groundspeak:attribute>).
People who write programs that parse XML code (like GPX files), generally will ignore tags whose values they don't recognize, at least if these aren't vital to the program's operation. Later, if they decide that these new values can be useful to the program, they will change the program to recognize these values and handle them accordingly. In the meantime, though, these new values will not cause their programs to break.