My 66 pound, 15-month old boxer boy played recently with a min-pin PUPPY... so teeny!! - and it ended up basically being my Guinness lying in the sand with the min-pin bouncing and jumping all over him (he was so patient, and so gentle). There are also pictures in my gallery of Guinness, at all of 12-13 pounds, playing around and with my sister's two fully-grown boxers (including a 60-pound female and 75-pound male). They just seemed to naturally know how to scale back their interactions not to harm the little one. AND, Guinness regularly plays with a little short-haired terrier... it's SO funny when they are running around, and the terrier runs UNDER Guinness - that tells you something about the size difference! So I'd say a friendly pair that includes a 40-pound dog and an 80-pound dog should be just fine!
One observation - I try never to have situations where one dog is on-leash and one off-leash. That can set them up for problems. Both on-leash or both off, but not one of each. Dogs have two options, basically, if a situation gets tense for whatever reason - flight or fight. If one is leashed, the "flight" option is removed and "fight" is their only recourse. NOT that friendly dogs are likely to fight, but it changes the balance of things, and I think it's always best to set them up for successful meetings even if their temperament makes a fight seem like a very unlikely possibility. Just some food for thought!
Check out my picture gallery if you want some reassurance on big dogs playing with smaller dogs.
Henri plays well with dogs that are under 10lb with no problems. They do seem to realise their own strength - and if they don't, the littler dogs soon lets them know about it.
