We've kinda got the same problem with our other dog, Dozer. He's not a big dog either (50lbs - big to some, but friends who've never seen him in person sometimes think he's a horse.) And when we photograph the dogs together it gets real confusing, real quick. Due to perspective, she sometimes ends up looking bigger than him.
What's my point? None, I suppose. But I've had this idea in mind to do a series of photographs of the dogs, with various reference points (alongside a human, compared to a hydrant, that kind of thing.) If I get around to it, keep an eye on Flickr, or here.
Oh, and other people must have the same problem with their doggey-pictures.