The German Duden explains bonsai by saying that it is a "Japanese dwarf tree" or "the art of pulling dwarf trees practiced in Japan". The view that bonsai are small is so widespread that the word bonsai is often used as a synonym for "small". The same you can find in the Oxford dictionary: Bonsai is "the art of growing ornamental, artificially dwarfed varieties of trees and shrubs in pots."
The opinion that bonsai are small or even dwarfish is wrong and is based on the ignorance of the writers. Bonsai is above all an artistically designed tree in a pot. It has nothing to do with size - as you can see on the next pictures.