Do you think maybe the reason Burns does not put weight on dogs is that it is not designed to? It is made to a vet's recipe (John Burns) and vets are rather like human doctors in that they do not like (excess) weight on dogs/people.
From what I have read on other threads, if you fed the recommended amount on most dried food, a lot of dogs would be overweight, but this does not seem to be the case with Burns, rather it seems you need to feed more - just an observation
don't have excess weight on my dog - he's 12.7kg which I think is fine, but my vet thinks he could have 1/2 a kg more on him! When he was on Burns he was skinny (for him, around 11kgs) and had dry fur and flaky skin. It just didn't agree with him. I truly believe it depends entirely on the individual dog and it doesn't suit all of them. Much happier on BARF and a bit of AG.
If you feed more Burns (like any food) than recommended you will either overfeed your dog which will cause diarrhea or they will expel more as their body will get rid of what it can't digest. If it's not getting what it should from the food anyway, then imho increasing the volume is not going to sort the problem.
I don't go much on marketing hype, and I feel there's a lot of that with Burns.