Nagash is even way bigger than his model would indicate. In the novels his giant form is just a manifestation he forms at will.
I don't think Sigmar's form is that much larger than the stormcast. During the scenes where they interact with him, I don't recall him being described as towering a 100 feet in the air or something. I think it might be more like the primarch size in
40k. After all, way back when
whfb and
40k launched it was implied he was a primarch. They have since gone in another direction though.