When I had GSDs, prior to about 14 yrs ago, I heard they were bred to herd sheep, not bred for the purpose of doing Schutzhund. This protection thing for humans and against humans is not what they were originally bred for, was it? Weren't they suppose to protect sheep? I know they make wonderful protection dogs, obviously. But as long as we're talking original purpose....
I think the Lab is suppose to be fairly sturdy, stocky, etc to enable it to push through thick brush in the wilderness of Canada when upland hunting. Of course, not obese and show winners of any breed need to be in good shape. What you're probably seeing is muscle. They're not only good water fowl retrievers, but good upland hunters. Look at a Newfoundland. Part of a Labrador's ancerstory is the Newfoundland.
My female Lab was rather small, shortish, fairly stocky, her head wasn't as blocky as I'd like. Her sire was a Canadian champion and field trial champion and was very, very stocky....short, strong, sturdy. Her mother was an American field trial champion and not as good looking.(IMO) She was not a conformational champ. She was a tad more the lanky look and a pointier, lighter head. Bonnie came out looking better than her mother and not as good as her father. LOL. This was a hobby breeder I think....not a back yard breeder, exactly. He was responsible in most ways but probably shouldn't have bred the female because her conformation wasn't impeccable. But she sure had a wonderful dispostition...sweet and exuberant. The dad was more laid back...really sweet. This guy wasn't in it for the money. He had scads of money anyhow. He spayed her after this litter and gave me back $100.00 if I had Bonnie spayed. LOL. This was about 14 yrs. ago.