You're right, we always hear these pairs of adjectives in those orders, but there's nothing grammatically wrong with switching the orders; it's just what has become customary. "Why do we say ... " "Please clarify this for me."

Actually, there are rules to the order of adjectives in English (though most English speakers don't learn them anymore, and so they are unaware of them). Here is the order, from first (left) to last (right): Determiner (e.g., a, an, the, these...) Quantity or number (e.g., three, many, some...) Quality or opinion (e.g., ugly, rude, pleasant...) Size (e.g., little, enormous, tall, short, microscopic...) Age (e.g., ancient, old, new, young, modern...) Shape (e.g., fat, round, baggy, bent...) Color (e.g., red, blue, dark, bright...) Proper adjective (e.g., Russian, Italian, American...) Purpose or qualifier (e.g., sports, pet, water...) (NOUN) This is why we say "tall and thin" or "short and fat": because size comes before shape.

There is nothing wrong with saying those phrases either way. The way you’ve always heard , “short and fat” is simply habit. The meaning is the same when you say “fat and short”.