A potential route to reduce CO2 emissions from heavy-duty trucks is to combine low-carbon fuels and a hybrid-electric powertrain to maximize overall efficiency. A hybrid electric powertrain can reduce the peak power required from the internal combustion engine, leading to opportunities to reduce the engine size but still meet vehicle performance requirements. Although engine downsizing in the light-duty sector can offer significant fuel economy savings mainly due to increased part-load efficiency, its benefits and downsides in heavy-duty engines are less clear. As there has been limited published research in this area to date, there is a lack of a standardized engine downsizing procedure. This paper uses an experimentally validated one-dimensional phenomenological combustion model in a commercial engine simulation software GT-SUITE™ alongside turbocharger scaling methods to develop downsized engines from a baseline 6cyl (2.1 L/cyl, 26 kW/L) pilot-ignition, direct-injection natural gas engine. Since there is a reduced power demand from the engine in the hybrid powertrain over transient drive cycles, this study compares two methodologies to achieve a 230 kW engine: a reduction in number of cylinders at fixed displacement (4cyl- 2.1 L/cyl) and a reduction in cylinder displacement volume but retaining six cylinders (6cyl-1.4 L/cyl). The power for the downsized engine is reduced compared to the baseline engine since a future hybrid powertrain will not need as much power as a non-hybrid. By retaining similar total displacement and equivalent power rating, the impacts of engine size reduction can be distinguished from the scaling of the turbocharging and air handling system. The engines are evaluated over a series of steady-state and transient cycles based on a reduced load duty cycle for an engine in a hybridized vehicle. The results indicated that, as expected, downsized engines demonstrate increased peak cylinder pressure, exhaust gas temperature, boost pressure, and turbocharger speed compared to the baseline engine when all engines undergo the same reduced load duty cycle. Distinctly, the 6 cyl-1.4 L/cyl variant showed increased heat losses due to the higher surface area to volume ratio in the combustion chamber, while the 4 cyl-2.1 L/cyl variant had higher exhaust enthalpy losses. Both downsized engines showed lower friction losses than the baseline engine. Due to these offsetting effects, neither of the downsized engines showed a significant improvement in brake specific fuel consumption (BSFC). The change in mass due to the smaller engine offers only a minor improvement in payload capacity compared to the reduction in the maximum torque.