Both of my daughters were 19.5 inches long. My first one was 9 pounds 9 ounces and my second one was a mere 5 pounds 6 ounces.
The difference: 1st pregnancy I consumed large quantities of protein by way of soy protein shakes and ate lots of healthy vegetables. I was also working and working out nearly every day up until 3 days before she was born. Diet and exercise
2nd pregnancy: No exercise. Not as much protein. Breastfed first baby up until I was 3 months pregnant when the pregnancy hormones put a stop to my normally abundant milk supply
Conception was only 20 weeks after the birth of my first child, so my body was nowhere near recovered.
1st, bigger baby, was born a mere 3 days past the EDD.
2nd, tiny baby, was born a week or so past the EDD and wasn't getting any bigger.