The lead weights are put into the saddle pad in order to add weight to that of the jockey so they meet the impost set forth by the racing secretary. The impost is a weight assigned in handicapping races in an attempt to equalize the chances for all of the horses in the race. "Better" horses are assigned higher imposts.
what is impost tax
The weight of professional jockeys ranges from 108 to 118 pounds. The jockey and the riding equipment cannot weigh more than 126 pounds.
Horse jockeys are typically shorter people. There is no average for their weight though it is doubtful they weigh much.
So that they don't slow the horses down.
They watch what they eat / drink, and get plenty of exercise.