Somewhat powerful iGPUs do require more silicon, but the design of that silicon is going to differ compared its discrete counterpart — whereas discrete GPUs are

Somewhat powerful iGPUs do require more silicon, but the design of that silicon is going to differ compared its discrete counterpart — whereas discrete GPUs are typically designed for most performance possible with the least silicon at the cost of heat and power usage, iGPUs are designed for perf per watt, as seen in Apple’s M-series SoCs. There’s probably some energy savings to be had from the GPU sharing system memory and moving data around less. On the software side of things, iGPU drivers tend to better optimized for efficiency compared to discrete GPU drivers, where that’s almost an afterthought and in some cases broken (like the laptop I had where the sleep state in the drivers for its Nvidia card was broken for years).