The reference of "thick with saddle muscles" is a crude description of the shape and strength of the women's thighs from frequent horseback riding. While astride a horse, both Western and English contemporary riding techniques require the rider to use their thighs to stabilize the body to the horse's movements. Personally, I wouldn't write "thick saddle muscles" or "thick with saddle muscles" as a description; however, "thick with saddle muscles" is grammatically correct.

"Saddle muscles" is an expression which describes the muscles developed in the thighs due to long experience with horse riding ( saddle = седло) . The author here is trying to describe the body shape of an experienced horsewoman ;) he used the expression "thick with saddle muscles" to emphasize on the idea that the thickness of her thighs is because of the frequent horse rising :)