Quote Originally Posted by The Ceej View Post
MORE EDITING:
According to this, this is why we were taught that:
At one time astronomers thought Pluto was bigger than Mercury. Now we know that Pluto is only half the diameter of Mercury.
It should be noted also that according to your linked article, this was disproved in the 1970's. You have stated that your textbook was from the 1990's.

It is possible that rather than being taught incorrectly, you have actually experienced some kind of problem when learning about planet sizes, such as poor memory, confusion, dyslexia, etc.