"big" hunting dogs ?

I don't really believe its neccesarily the size of the dog that hiders endurance as much as it is when folks breed for size they get the bulk and sloppy confrmation in the dog. The dogs joints must be able to handle the weight it is carrying on them, fluid movement is key, and balance in structure.
Face it alot of the larger breeds have the crap bred out of them to look cool and tuff and not as much emphasis on the health and ability of the dog. The smaller dogs can usually carrier their weight and have the strength for agile movement, because they are not as "bulky"

The larger dogs will usually have more joint and health issues, because attention is not paid to breeding healthy hips and joints, they are not put to any kind of endurance or agile, or duration test before breeding them.
The thick American Bulldogs are a good example, for short distance they are really strong, but they are two thick and heavy with not enough lenghth of muzzle to get enough air/oxygen through them to go for endurance.

What is the longevity of the breed of dogs you are crossing in? Can they still run 10-20 miles at the age of 6-7 years with good speed and still good the next day with out being sore and stove up?
Great Danes are not known for long life spans, usually the greater the size of the dog, the shorter the life span, and the slower the maturity age, so when it comes to breeding dogs for size, It would be a great assett to consider the particualr dog you are using to cross into the line as to "are you breeding a dog that will be a dog that has a shelf life of hunting for only two to three years before it has reached its potential and then past its prime?"

Personally I feel after feeding and raising one up to a year of age, then put another year of grooming them into a top dog, and then I want to see them excell for several more years after that before they are the downhill slope.
I like a big dog, but over 80 lbs I think you are more likely to get less longevity of a career from a working dog.