But a good part of it is the breeding. Conformation (show) Labs and field (hunt test/field trial) Labs have diverged a good deal in body type and in activity level. Field Labs are slender and sleek not only because they are "on the go" all the time, but because they are bred for that "on the go" temperament AND a lean body type.
My dog is an odd bird, because she is one half conformation and one half field. Her dad is a simply gorgeous conformation Champion, who probably couldn't find a bird in a phone booth if you gave him printed instructions, and whose idea of exercise is a trot across a nicely mowed green lawn to pick up a tennis ball. Her mom on the other hand is the offspring of the only Chocolate Lab ever to win the National Field Champion title, bred to a Chocolate from the best field kennel in the country. She was the puppy most like her mother -- but she still is sturdier in build and wound a little less tight than Tori. Here they are exiting the very same tunnel, from the very same angle:
But people think Shelley is skinny . . . until they meet Tori.