Cut a chapter when removing it changes nothing that follows. A pleasant scene, clever exchange, or interesting detour may still be expendable if it creates no new obligation, information, cost, or relationship state.
Combine chapters when they perform similar dramatic functions. Two separate investigations can become one sequence in which the second discovery contradicts the first. Two relationship conversations can become one encounter where the characters want incompatible outcomes. Combination often creates pressure because the protagonist no longer gets time to recover between developments.
Deepen a chapter when its premise belongs but its consequence is too small. Let the discovery implicate an ally, make the victory expose the protagonist, or allow the apparent solution to create a moral cost. Deepening is often better than cutting when the chapter carries important character or thematic material that has not yet been connected to the central engine.