Mass is a calculation of precise weight. The mass of a medium chicken is guess work at best and there are over 175 breeds of chicken all with different characteristics and variants.
In market terms, a broiler or fryer is a young meat-type chicken that can be cooked tender by broiling or frying and usually weighing between 2 1/2 and 3 1/2 pounds. A roaster is a young meat-type chicken that can be cooked tender by roasting and usually weighing 4 pounds or more. A stewing chicken, hen, or fowl is a mature female chicken, often the by-product of egg production, with meat less tender than that of a roaster and can be cooked tender by stewing or a similar method will weigh from 4.5 to 8 lbs +.