This would be the case for any lossy format with decent compression. We are much less sensitive to color than we are to brightness so we can compress color much

This would be the case for any lossy format with decent compression. We are much less sensitive to color than we are to brightness so we can compress color much more than we can compress brightness for the same perceived quality. Not only that but color and brightness are often correlated, and advanced compression techniques use that property, see "chroma from luma" in AV1. Without compression, indeed, it can be 3x, or even 4x (for alignment).