What is up with the replay files size?
I read and it says only a handful of KB.
How does that work out, is only the control input saved and then the replay is rendered real-time, because I can't think of any other method.
I doubt it just saves the control input as there are more factors than just the input in what the cars do on the track. If it does save primarily the input in the race to give you a real time race it also will need to remember the car its tuning and (in the case of races) the lineup and all their tunings and input. It's more likely to be an actual file of video that is compressed.
It's indeed the control input, the car specs (parts & setup) and the possible AI information. There's no way that any video file could be compressed that much. Not to mention that the method is proven when a replay made with a PS2 is watched with a PS3 - backwards compatibility needed of course - as the timing is a couple of milliseconds out of sync and as a result the car will go off the course sooner or later. Usually sooner. As a (not so) interesting fact a wheel replay file is usually larger than a pad replay file as there's a lot more data thanks to all the small movements.
