Jockeys must be light to ride at the weights which are assigned to their mounts. There are horse carrying weight limits, that are set by racing authorities. The Kentucky Derby, for example, has a weight limit of 126 lb (57 kg) including the jockey's equipment. The weight of a jockey usually ranges from 108 to 118 lb (49 to 54 kg). Despite their light weight, they must be able to control a horse that is moving at 40 mph (64 km/h) and weighs 1,200 lb (540 kg). Though there is no height limit for jockeys, they are usually fairly short due to the weight limits. Jockeys typically stand around 4 ft 10 in (147 cm) to 5 ft 6 in (168 cm).

What's the best answer to this question: do jockeys have to weigh a certain amount?
yes