In contrast to the Inca Trail, horses and mules are allowed on the Salkantay Trek. These carry additional luggage of the hikers besides the camping equipment.
It should be possible to hand over a bag with 5 to 7 kilos to the horsemen. You should check with the trekking agency if this is really the case.
In the extra luggage, you should carry a sleeping bag and the clothes you don't need during the day. The guides are usually at the campsite much earlier to set up the tents.
Each trekker carries a daypack with water, rain jacket, camera, sunscreen and some snacks if needed.