When I was little I saw a spider that was about the size of a football, and at five years old, I thought it was better to show the size with my hands rather than try to explain the size. So I said the spider was "THIS BIG" with my arms extended as far as they could go.
I don't think anyone else thought the spider was really "THIS BIG", but in my mind it really was!
Ever since then, whenever my family wants to over-exaggerate the size of something, they say it is "THIS BIG", which has stuck with us ever since.