I bet you that if Silva fought GSP at 178(this is the number that keeps popping up and makes sense) then the actual weight difference fight night would be negligible and that both fighters would come in weighing 190-195 fight night.
The weight difference doesn't even matter though, even if Silva has 10 lbs on him fight night it won't change the outcome of this fight. Silva doesn't win fights because of his size, wrestling etc and 10 lbs of water isn't going to be the difference in him being able to stuff GSP's takedowns.
The heigh factor may be an issue but GSP first has to fight Condit who is the same height as Silva. Honestly the taller height if anything will make it easier for GSP to take Silva down. On the ground Silva's lanky legs will make things tough but he will also be against one of the very best fighters for passing guard.