@PennyChan,
Not sure of the origins of the term big box hardware store but it describes one of the very large major hardware stores (large in terms of massive amounts of square footage per physical store). The stores are monstrously large like warehouses.
Big box hardware store (130,000 square feet of selling space) as opposed to mom and pop hardware stores which might mean 20,000 square feet of selling space or even far less.
Big box hardware stores are nationally or globally owned corporations where they can sell their goods for much cheaper due to their buying power.