Just to give you an example. My neighbor has two yellow labs. A 3½ year old male that tips the scales at 92 lbs (all muscle) and a female 9 years that runs roughly 65-70 lbs. Even though he is quite a bit younger, she can normally run circles around him when they are playing. When tossing a bumper she normally gets to it first unless she gives him one every now and then..During the summer months you can easily see he tires much more quickly because of his size.