We don't personally have a big and small pair (mine are both big!
) but I do have friends at the dog park who have a small/big pair. There is a lady who has a 4-5lb Yorkie and a 40 or 50lb Bassett Hound. They play together a lot. There is also a lady with a 4-5lb Pom/Chihuahua mix with a larger (maybe 30lbs?) cocker spaniel. My friend has a 7lb Dachshund who loves to play with her 100lb Golden Retrievers.
I think it's fine given they are supervised during rough play, of course.