The light breeds tend to be egg layers only and aren’t renowned for their eating qualities, but many of the heavy breeds are known as dual-purpose breeds as they are good for both laying and eating.
For example, the original Light Sussex and Rhode Island Reds, or a cross of the two breeds, were developed in the early 1900s as dual-purpose birds, and some strains were also excellent layers.
In the 1940s, lighter Leghorn hybrids were developed for the production of table eggs as both intensive housing and commercially-made feed became popular. These birds were carefully bred for their feed-to-egg conversion rate (eg, smallest amount of feed for the most eggs possible), not for longevity, which is why commercial hybrids don’t tend to be long-lived, but in their first 1-2 years are outstanding layers.
Some heavy breeds are very poor layers, their genetics working to lay down a great deal of muscle, like the Cornish
and Indian Game breeds.
The creation of the commercial layer and the broiler chicken has meant dualpurpose breeds have lost favour, unable to compete against specialist egg layer and meat birds selected for the extreme ends of productivity, as has happened with dairy and beef cattle breeding.
Selection for growth and productivity is often the opposite to fertility so high egg producers are fertile and high meat producers are not-so-fertile.
Along with a difference in body size, there is a big difference in metabolism and the nutritional requirements for these birds to function at their most efficient. Knowledge of the nutritional needs of poultry has come a long way in the last 100 or so years, and more rapidly in the last 50 years. There is a greater understanding of the nutrient requirements of the birds, and the ability to analyse the make-up of foodstuffs and to test each seasonal batch of ingredients for protein and energy levels. This has meant nutritionists can formulate diets to suit both the age and type of stock (eg the efficient diet of the egg layer vs the higher protein diet of the meat producer).