And do you know why it was only 600 kb? If you're familiar with MSTS, go and have a look in the World folder of one of your routes, and you'll see that the file size is, on average, only marginaly larger - and yet the entire route size can take up hundreds of mb. Why do you ask? Because the baseboards in Trainz and the world tiles in MSTS are exactly the same - just vector data, telling us the terrain details, object placement etc...it's really the objects themselves, with the numerous textures that total the route size up.
Go into your World
Downloads -> folder, and you'll notice that the size of this folder is huge. Mine is a meagre 136mb in size, with only 80 odd downloads in there, and is bound to increase dramatically by the time Vic Thoroughbreds is finnished and beta installed on my box.
Anyway, if you were to combine the size of all the assets used on your layout with the baseboards themselves, you'd soon find out that it's quite in excess of the 600kb you quoted as the filesize of your route