You may think you do get more luggage weight allowance since you get upgraded to first class by using your miles for that flight. However, I have an itinerary that is very long (5 flights, containing both domestic and international), and I upgrade the third flight (domestic). Now, all of the flights except the last international one allow me to check-in 70 pounds-per-piece luggage.
I'm confused. Anybody knows a formal reason for this?