It may have converted it to trackpoints, most Garmin GPSes have a trackpoint limit of 10,000. Trackpoints are just a log of a track. They don't interact with the routing system, or allow naming.
I've never tried navigating based on one, and I'm not sure if the arrow will keep pointing to the next point on the track.
So, imagine you've done the route once, if you look at the map page you'll see the log of where you've been. This is exactly what you get when you upload a tracklog. The idea is you follow the track on the screen to keep you on route.
(And, as I understand it, if you upload a track of 900 points you'll only have 9,100 trackpoints available for saving the tracklog of your actual ride).
[EDIT] If it was trackpoints then gpsbabel gives a way of filtering the number of trackpoints down to a more sensible number. See the gpsbabel docs...