I have had several quite funny (to me) anecdotes where I remember something vastly bigger (buildings that are "exactly the same but twice the height", huuuge houses from friends' parents, food being humongous, etc.). Don't underestimate the fact that as an adult you're about twice the size as when you were a kid learning to write! :)

That's why I estimate the diameter as 1.5X even though my memory feels more like 2X. In my case I had the larger pencils alongside regular-sized color pencils in my pencil case, so I do remember them being distinctly larger.