Why are horse jockeys so small?

[lumos]

"In flat racing, each horse carries an assigned weight. This weight includes the weight of the jockey; his/her riding gear, and the saddle and undergirth. Assigned weights are usually low." This is what I found on the subject.

[DeeDeeDeVour]

The smaller the rider, the lighter the load and, therefore, the more able the horse.

[FrenchyRomain]

you do realize : the lighter the rider the faster the horse right?
so the smaller the dude riding the horse... (finish that sentence?)

[Gedaria]

It cannot be weight they all weigh the same..

[jakeofalltrades]

so the horse runs faster

[DFENS]

How do you not know this?

[Cryptic-Game]

So the horse can run faster.