I could try smaller eyes. It would make them feel more "massive", proportionally speaking, and that would make them more intimidating.
Those particular heads were built strictly on the proportions of a normal human skull. But if the skeleton of the koloss grows, then it makes sense that the skull would as well, and yet that doesn't necessarily mean that organs like the eyes would grow in equal proportion...
You know, I think you're right. On a "mature" koloss, at least, the eyes could very well be smaller. I'll keep it in mind, 'cause I'm pretty certain this won't be the last time I draw them.
I'm pretty much always open to feedback. Others catch details in the text that I miss, and getting a well-rounded input on the various ways in which different people interpret the same text is beneficial to creating a design that accurately represents not just what Brandon describes, but what the audience perceives in that description.