Do grayscale images take less space?

Grayscale images typically occupy less space compared to sRGB images due to the simpler representation of each pixel. However, aggressive optimizations on the chroma channels in sRGB images can sometimes make a 4:2:0-subsampled image close in size to a grayscale image. The Y'CbCr color space is used internally in formats like JPEG/JFIF, allowing for separate optimization of the luma (brightness) and chroma (color) channels. Subsampling techniques like 4:2:0 significantly reduce file size by prioritizing luminance over color information, leveraging the human eye's sensitivity to brightness changes over color changes. While grayscale images are efficient in size, they do not offer significant gains over optimized sRGB images subsampled to 4:2:0. This optimization balance ensures reduced file sizes without compromising visual quality, making it a common practice for web image distribution. Subsampling plays a crucial role in determining the final image size, with 4:2:0 being a prevalent choice due to its effective balance between compression and visual fidelity.