Any help would be wonderful! I own a female black lab which is seven months old.  Shortly after purchasing my dog, several friends also researched and brought labs into their homes. After getting the dogs together, I was amazed at the differences in structure of the four registered, closely aged labs. The group consists of 3 females and a male.  The male, as expected is a little larger but even with the females there are huge differences.  My dog is on the small side with an abundance of energy and not much of an appetite (I took her to three vets to check into that but she's cool) while the other two females are huge and wolf down their meals.  At first I thought I was underfeeding but when I shared the breeders feeding recommendation with the vet he said it was plenty.  With all the jokes about the little black "greyhound-duck" I decided to do a little research.  In some places I read that there was a show line and a field line, but the info was few and far between.  If anyone knows any info or were I can look to clear this up I would be grateful

Just look around you at the humans you pass in the street. How many of them look the same? Look at pictures of Labrador Champions, there are so many variations.

As to "Show Lines" and "Working Lines", a working Labrador is selected for it's abilities in the field. One of those abilities is an athleticism such as not common with a show Labrador so it is not unusual for them to be leaner looking.

Regards, John