Heavy and light body impression materials are used with:

Elastomeric impression materials: They are supplied in two components, a base paste and a catalyst paste (or liquid) that are mixed before making impressions. They are often formulated in several consistencies, including extra low, low, medium, heavy, and putty, in increasing order of filler content. Extra-low and putty forms are available only for condensation and addition silicones. Polysulfide is provided only in light-body and heavy-body consistencies. There is no heavy-body product for condensation silicone. Pigments are added to give each material a distinct color. The base paste, is a polysulfide polymer that contains a multifunctional mercaptan (-SH) called a polysulfide polymer, a suitable  filler  (such as lithopone or titanium dioxide) to provide the required strength, a plasticizer (such as dibutyl phthalate) to confer the appropriate viscosity to the paste, and a small quantity of sulfur, approximately 0.5%, as an accelerator. Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 153