I'm a long time Tyre user, and here are the things I've found when trying to work with one of these really large .gpx files.
If you download a .gpx file from an online source, like the MABDR file or the one here that Chirosyd is dealing with, and you try and open it with Tyre, you don't get the option of whether to open it using waypoints, route points, or route tracks; that "import" menu doesn't appear at all, and the route simply opens. Tyre has a maximum waypoint limit of 1000 waypoints, so that's how big these huge files end up being.
If you open the file and re-save it under a new name, and then open the renamed file in Tyre, you do get the import menu that allows you to import the file as either waypoints, route points, or route tracks. However, regardless of whether you open the file in waypoints, route points, or route tracks, the outcome is the same; a file containing 1000 waypoints (the maximum allowed by Tyre). My Garmin Nuvi has a file size limit; I don't know what it is precisely, but it's less that 468 points per route. My Garmin can not read one of these huge unedited .gpx files, and I don't get the option on my particular Garmin to choose if a route opens with waypoints, route points or route tracks.
So at this point I have two choices: I can chop up that thousand waypoint route into five or six smaller segments, or I can edit the route to a manageable size that my Garmin can accept. For me, it seems silly to have to break up a 270 mile ride into five or six segments just so my Garmin can read it, and that gets a lot worse if you're talking about something like the MABDR, or a cross country route; in fact, the MABDR route had so many points that even Tyre wouldn't open it correctly.
My solution for these large .gpx files has been to open the file on Tyre and edit the number of waypoints down to a manageable number. Basically you open the route on Tyre, and then stop the route from calculating as soon as the waypoints load (if you let it keep calculating the route as you edit the waypoints, it makes the computer hang up and sometimes even crashes the program). Then I basically just delete huge chunks of those waypoints, leaving enough to shape the route correctly. I edited the Romney sport touring route this morning, and it now contains 155 waypoints. That will easily load onto my Nuvi as a single route, with no need to break anything into segments. When I edited the MABDR route, it initially contained around 6000 waypoints; my edited version has 468. I still had to break that into segments, because my Nuvi can't read a route with 468 waypoints, but that was acceptable for a route that was over a thousand miles long. It took me about 25 minutes to edit the Romney sport touring ride down to a manageable size. It's boring, to be sure, but it doesn't take much time, and it's been a workable solution for me if I want to use a .gpx file created by someone else.
There may well be other ways of doing it that work better than mine, but this is the way I've figured out that works for me.