50mm nails costs 6 pound 50 pence per kilo and longer 75mm nails cost 5 pound 40 pence per kilo suggest why the longer nails have a lower cost?

50mm nails costs 6 pound 50 pence per kilo and longer 75mm nails cost 5 pound 40 pence per kilo suggest why the longer nails have a lower cost?
manufacturing costs, you don't need to make as many 75mm nails per kilo