The horse that lost the jockey, yes I saw that race.
The reason that it only wins by a small margin is for two reasons.
First is pace, there is nobody holding it back so it runs on pace but may not be an on pace horse, but no weight at all sees them often win.
Second, they tend to veer all over the place in the head on replays and do lose a bit of ground. Think it was Bossy who said Melbourne races are run at a different style of pace. It's a scramble, then a really slow middle sectional and then a sprint. (according to him). He said you have to ride differently here. Speedy is also right, the horse will run with the pack, even if able to win by half the length of the straight, horses are more comfortable amongst the herd.
It's the jockey that pushes the horse out of it's comfort zone.
As to weight in training versus race day, many heavier track riders are used to get the horse fit, but jockeys are used mostly in fast work and barrier trials. Certain trainers don't need heavier jockeys on their private tracks, they run them on an incline, which has the same effect as a heavier person.
Others use machines with belts that mechanically rise on an incline with fans going to cool the horse and simulate wind.
Here's a very good example:
https://youtu.be/Ftm-qZazMyQ