@Thunderstrike: Asking some very good questions.
To ask such questions you must already have some fair knowledge in the matter.
I have a suspicion that you are just testing your own knowledge , without revealing it to others.
So be it. This is a dog eat dog world.
Racehorses carry weights as assigned by the handicapper. The handicapper allots the weights based on comparative rating. Ratings go up or down depending upon the performance of the horse. That means the form. But form is temporary. Class of the horse is permanant. The handicapper is guided mainly by the form of the horse. He is not overly concerned about the inherent class of the horse or its performance in distances unsuitable to it or by the ineptitude of the calibre of the rider. That is where the trainers' acumen in placement of the horses in races of different distances at different weights with different jockeys comes into play.
The horse carries the weight in different ways : the live weight and the dead weight.
A horse assigned a weight of 60 kgs if ridden by John or Srinath or Antonyraj , 56 kgs is contributed by the weight of the jockey, the live weight. The rest 4 kg is by the lead weight inserted into the saddle pouch - the dead weight.
Now suppose the same horse carries a 47 kg jockey. The dead weight will increase to 13 kgs.
John or Srinath or Antonyraj are skillful riders. Their weight will not hamper the performance of the horse , rather their skills will contribute a lot. But the dead weight does not contribute anything other than dragging down the performance .
A 500 kg horse can easily carry 60 kgs compared to 390 kg horse. Specially if the horse runs in a class down - a class 4 horse running in class 5 for example. More particularly in sprints ranging from 1000 meters to 1400 meters. Longer distances ? Yes horse body weight and handicap weight may matter if the distances are longer.
Exceptions like Juliet , a light weight mare , that can carry weights despite distance. She is a exception. That is one reason that she has endeared herself to one and all cutting across the rivalries. For Juliet , age , weight and distances don't matter. She just loves to race,to compete and to show her mettle at every oppurtunity.
When looking at the weights , look at the weight of the horse, weight of the jockey , distance to be covered, the running styles of the horse , the skills of the jockey etc.
A lot of factors determin the performance. If you look for individual trees you will completely miss the forest.