Differences in more weather tolerant dogs within the same breed?

Sorry, my brain is a bit scattered in this post.
I see this topic all the time in working/sporting dog groups, primarily people being very critical of dogs with lesser heat tolerance than others.

So what are the differences between a more heat/cold tolerant dog within the same breed?
Why are some Aussies (or any breed, just comparing two dogs of the same breed) good in the heat while others are not?
Two dogs of one breed can look relatively similar, same coat type, snout, nares, muscle mass/weight, yet they differ in weather tolerance.
Does this come down to physical traits we cannot see on the outside or is it more to do with a dogs mind?
Is it simply a matter of self regulation? If so, then why would some dogs need more breaks than others?

Again, I am comparing dogs in the same breed, not comparing like a Vizsla to a Bulldog because the physical differences are to great to ask such a question.