I have four Labs, and they are all under 70 pounds. This is not only an advantage in the house, but in a boat.....they take up less space.
Generally speaking , most of the larger Labs I seen are more difficult to keep in shape, and they don't handle the heat as well (bulkier body.....more to cool off). However, another important part of the equation is most of the Labs I've seen are really not conditioned properly.....especially for the upland. Running retrieving setups, drills and cold blinds do not translate to the aerobics required in upland hunting.
Of my four Labs the smallest and slightest built seems to have more speed, endurance and staying power for the long haul. She loves to run out in front of the ATV. Of course a lot of what I see has to do with her personality....drive and desire.
So if I were looking to buy a Lab for the uplands, the parents would have to
not be large, have natural, quick and easy speed, move with a free, long stride, have a ton of go in their personality and demonstrate a love to run.....out front.
Oh, I forgot to mention....if they love to run big....it would be better if they pointed.
"Daisy"