Normally, when a breeder gives advice to someone to use a 20 gal long tank, it's to someone that just bought a snake and doesn't know much about them. They usually have one or two snakes and plenty of room. If you were to measure the floor space of a 20 long and a sweater box, you'd find the are very similar.
It's by no means a 'do as I say, not as I do' scenario. Breeders need something that is practical more then esthetic.