This is really a lot, I'm not sure that you really need all these huge textures all loaded at the same time.
Don't forget that your graphics card's resources are also used by the OS and other apps, that the amount of textures or the memory dedicated to them may be limited by the driver, that the available memory may be less than 6 GB because of fragmentation, etc.
If the driver says "out of memory", I'm pretty sure you can trust it