Well GSP trains year round, so that's not an accurate comparison. Of course Silva isn't going to be around his natural weight when cutting. Neither is GSP.
Silva has a significant size advantage over GSP. I'm surprised people even argue this.
Silva has a noticeable height advantage and a slight reach advantage. I do not equate either of these to be a significant size advantage.
The only way Silva's size helps him against GSP is with longer arms to strike and keep GSP at distance and with his longer legs to help defend himself while on the ground.
If Silva wins it is due to superior striking or BJJ. I highly doubt it will be due to his size in other words I highly doubt Silva will manhandle GSP stuffing his takedowns and throwing him around the ring... Especially since at a catch weight of 178-180 Silva will probably come in slightly considering he hasn't cut that far in years.
As for Silva's 202 weight. I think that was his natural weight at that time. He was coming off a rather recent fight and would have still been in close to 185 weight shape.