You forget one thing though... How much detail did Red Dead Redemption have in it? Not as much as GTA IV has. Considering that 80% of the map in RDR was mainly geomentry work with hills, mountains and rivers. There was many towns in RDR, but not big enough to consider as much detail. So they were able to create a massive map for the game considering that they only used small models for buildings and they were well spaced out with the towns being in different area's. The grass and plant life would probably not take up that much resource. It is a shame that they never released it on the PC because it was no "Viable". Article Here.
You got to remember that PC games can only handle so much, but console games can go so much further than PC games. You got to take in to account of the millions of different PC hardware configurations and the graphics card compatibility.
So you kinda get stuck there.
The game was filled with lots of Trees and grass and that takes a lot of resources.
-waR'.se- I don't know who you are, where you are from, or what you want... But if you threaten my freedom... I'll kill you.