Well I've caught 18" crappie before and I've even caught one that was 20" back when I was a young lad on a big white rooster tail. I didn't know back then how big it actually was. I've always looked at it from the inch standpoint but actually a 20" fish can be really skinny and not weigh much. Other side of that is a 16 or 17"er can be fat and weigh big.