A heifer is a female bovine that hasn't had a baby calf yet. Basically, she's a teenage cow.
Cows generally have their first baby calf when they are about two years old. We commonly call cows in their first lactation (who have had their first baby) as a first calf heifer. They are technically a cow now. Just something we do.
Like teenage people, these heifers are growing a lot and utilizing the feed they eat for growth. They grow a lot! A cow's weight depends a lot on her genetics and her breed, but an average calf can be about 90 or so pounds when it's born. Some calves are larger (just like some human babies) and some are smaller. Grown cows can weigh 1200-1500 pounds. Jerseys are the little brown cows and tend to be smaller, perhaps around 900 pounds.
Regardless, they are large animals! They require a lot of food regardless of whether they are growing during their heifer years or producing milk as a cow.