Malyk (My RR) eats what he is supposed to according to the bag and Baloo only eats a half cup more than suggested on the bag. Chevelle my APBT is the only one that really eats more than she should. (Although by her activity level, I'm not surprised by how much she eats.)
He may not look stressed about being in his crate while you are around and he may go in on his own, but when he is closed in his crate and you are not around it may be a different story. A lot of dogs get very anxious when closed into a small space and they completely lose their mind!
I would *almost* bet your other dogs have Giardia, and just weren't emitting it in the fecal you sent in for testing. When I had a dog who tested positive, my vet get me enough stuff to treat everyone because it DOES spread so easily.
Hope treating for the issues solves everything for the poor pup!
I went through it with a pup from a breeder when I was still breeding, we treated the whole house too, I forgot to mention that... Even the affected pup passed her first fecal, they caught it on the second