How does 1kg of lead have the same density as 1kg of polystyrene?

Both 1kg of lead and 1kg of polystyrene have the same mass, despite lead being much denser than polystyrene. Density is defined by mass divided by volume, so if the mass is the same, the volume must differ to maintain the same density. In this case, the volume of lead will be significantly smaller than the volume of polystyrene, making them have the same density.