Which word would best describe the act of making something smaller in size or volume?

The word that best describes the act of making something smaller in size or volume is "condense." Condensation is the process where a substance transitions from a gas to a liquid, often resulting in a decrease in volume. This term is widely used in various scientific contexts, such as in chemistry and physics, to describe how gases turn into liquids and occupy less space.

While "compress" can also refer to reducing volume, it generally implies applying a force that causes particles to come closer together rather than a straightforward change in state or size like condensation. "Expand" and "inflate" have the opposite meanings, indicating an increase in size or volume. Hence, "condense" is the most precise term for the specific action of making something smaller in size or volume.