In split-cycle engines and air hybrid split-cycle engines, the sizing of the crossover passage is critical to engine efficiency. Efficiency can be improved by sizing the crossover passage volume to be small relative to the volume of the cylinders, and in particular relative to the volume of the compression cylinder. This allows for a higher pressure in the crossover passage, which extends the duration of sonic flow from the crossover passage into the expansion cylinder and increases combustion pressure. The methods, systems, and devices disclosed herein generally involve sizing the crossover passages, cylinders, or other components of a split-cycle engine or air hybrid split-cycle engine to improve efficiency.