The weight of ground covers vary based upon the material, the size of the material, and the moisture content. Whether or not it has rained in the past few days plays a big part in the weight of soils and mulches.
Generally, mulches weigh about 500lbs per cubic yard, but can go up past 800lbs once wet. Topsoil and compost weigh in at about 1200lbs per cubic yard, but they also hold water and can weigh as much as 2000lbs once wet. Last but not least, sand and gravels can tip the scale at about 2600-3000lbs per cubic yard.
Always check the payload capacity in your vehicle prior to being loaded up. It may appear that you can fit more product into your truck, but you want to ensure that you are not loading it past its allowance.