Toyota has developed more efficent engines technical designs
Toyota Develops Engines with Improved Thermal, Fuel Efficiency | TOYOTA Global Newsroom
The new engines to come will be 1.3 liter and 1.0 liter 4 cylinders
I know the current Gen 3 Prius has a 1.8 liter atkinson engine and the newer design for hybirds may be (not for sure) the 1.3 liter. There will 14 different finish product line.