Map of John Muir trail.
http://www.mappingsupport.com/p/gmap4.ph..._Muir_Trail.gpx

I found that data file here:
http://parkaymaps.110mb.com/

If the same track is in a GPX file and a KML file, the GPX file will always be bigger.

Gmap4 converts GPX files to KML files before displaying the map (required by the Google Maps API)

Google has limits on the size of KML file that can be displayed. Those limits are:
Maximum fetched file size (raw KML, raw GeoRSS, or compressed KMZ) 3MB
Maximum uncompressed KML file size 10MB
Maximum number of Network Links 10
Maximum number of total document-wide features 1,000

1 GPS track is only 1 "feature".