To "downsize the engine" means to reduce its size or capacity. This can be done to make the engine smaller, lighter, and more efficient without sacrificing its performance. Full definition
The current generation, or F10, BMW M5 was the first to feature downsized the engines with turbo chargers. (nashvillechatterclass.com)
This is the first of Kia's upcoming range of downsized engines to make its debut in Europe. (greencarcongress.com)
The eleventh generation of a compact car from Opel is a eal breakthrough as it comes equipped with new downsized engines and a complete new architecture. (carsession.com)