I think, even within a type, it’s partly luck how heavy set they are.
We have had three show type labs. All have been physically and temperamentally slightly different.
1- Both parents had JW and mum was ShC. He was the hardest work to keep slim. At his best he was 32kg, when I took my eye off the ball he crept up to 36kg! He was heavier in bone, but that 4kg was fat- my fault. We did do some showing with him as a young dog, he was on the finer side, and slim side compared to others. He was very chilled at home and out and about. The nicest ‘person’ you could wish for. We did some young dog pet agility and he wasn’t very good but we both had a nice time!
2- Both parents show dogs, mum didn’t have any titles, dad was ShC. She is probably the most conformationally correct of the three. Yes, left to her own devices she’d eat herself into obesity, but compared to dog 1, she’s much easier to keep a healthy weight, ranging between 25.5, to 27kg now at almost 12 and slowing down in terms of exercise. (Obviously if we’d have wanted to show she would have looked very out of place at her slim weight, however, several people involved in showing and one that had crossed from showing to working, said she was a VERY good example in terms of conformation). She has been the most intelligent of the 3, and easiest to train/most obedient. We did a few small pet gundog classes and she was great and had a lovely time. Had they been closer (1.5hrs away) we would have kept it up. We also did pet agility (and pet obedience) with her when she was young, just for fun. She loved it, and really wanted to do well. She’s a delight at home and has always been (once past the land shark puppy stage!)
3- She is the niece of dog 2. Her mum didn’t have any titles. Her dad is a ShC and has his show gun dog certificate (can’t remember the official title of it now- it’s been many years since we looked into it all). The idea with that certificate is to show that not only does a particular dog stick to the breed standard in terms of looks, but that they have the temperament and breed traits to do the job too. She is the finest boned of the three. She was second pick of the litter, in terms of aiming towards showing. However, I think she’s too fine boned to have done well, and the angles of her back end aren’t 100% if I were to be critical. She is the bounciest of the three. Despite her dad’s achievements she doesn’t have the temperament to have worked. Her drive to retrieve is not as strong as dog 2, nor is her desire to please. (Don’t get me wrong, compared to many many dogs, esp other breeds, she wants praise, but dog 2 is DESPERATE to get it right and be told she’s a good girl, and never needed a stern voice, whereas dog 3 does the canine equivalent of shrugging her shoulders and asking what’s next when she was told off). She’s super fun but also VERY cuddly. She’s 7.5 but we still get asked if she’s a young dog- I think the bounciness plus being fine boned and slim make her look younger than she is. Everything is ‘wooohooo!! My favourite thing ever!!’
My reply has got a bit long winded but what I was aiming for was to say I’ve had three show bred labs, the second two very closely related, and all related by the great and great great grandparents and all three have been different. Only the first one would I say was typically ‘square’, and not much unless he got overweight. However, he had to be on a MUCH stricter diet than the other two and, despite this, carried more body fat. I am in contact with a few owners of siblings and close relatives of 2 and 3 and they do vary amongst themselves as to how much bone they have as well as fat they carry.
All three would have been physically fit enough/capable to do pet/fun agility. I think only dog 2 would have had the drive to be competitive, because she so wants to get everything right.