This is by far the best answer possible.
Here's some bullshit for you.
Sport specific training, its not any different than anything else. Generally you are doing lots of repetitions of things you do in the sport, you might do supplementals of things that you know objectively are deficient. For example, beyond beginners and low midrange shooting, doing direct grip training is probably a complete waste of time. If you are crushing all the time when you are dry firing and shooting, and you are griping the gun hundreds/thousands of times a day already, then you are already doing this training volume and more is not necessarily better and is sometimes detrimental. Unless you find that your grip is actually breaking while shooting from fatigue or whatever there is little point in doing supplemental grip training.
Not being overweight is probably always good, but as you point out there are plenty of samples of big boys that do just fine. I think though the population is to small to really make the conclusion that size doesn't matter though. There are plenty of big boys that are also highly athletic despite appearance, but who gives a shit.