Chaos Knights are considerably larger than regular cavalry. Even the base size is larger. Pictured is a new Chaos Knight along with a Lumineth Dawn Rider. The tile in the back was used to keep the bases flush with each other.
I also have the old Chaos Knights. Comparing the new and old Chaos Knights there is a size difference, but I personally wouldn't feel bad if I was to mix them as I don't think it reads as out of scale so much as larger and a smaller riders/mounts.