Hi there, I’ve seen the answers about that it is normal for one side with the mastoid bone to be slightly larger than the other one, as not everybody has perfect body symmetry. However, my right-side mastoid bone is approx. 3x as big in size as the one on my left side. No ailing side effects (not that I’m aware of), I’ve never really inquired about it, because I have a medical professional in my family and they thought it was nothing to be worried. I’m not worried, more like curious. I don’t know if this is a rare thing or not.