Once upon a time photographers used to shoot on film. (They still do!) They used different kinds of film and one of decisions they made was the size of the negative. Pictures were not made up of a regular grid of square pixels but consisted of tiny grains of silver.
If photographers wanted more resolution, they needed a larger negative. It was rather straightforward: a negative that was 8″ x 10″ (yes, you heard it right, 8 by 10 inches – it’s called large format photography and it still exists!) by definition contained four times as many silver grains as a negative that was ‘only’ 4″ x 5″. Four times bigger, four times the resolution. And four times the cost.