Would agree with the above re: Burns - Misha was having double the daily ration just to keep weight on her. She is also quite small - under 25cm and weighs 8kg (she's 19 months old). Now that she's had her first season and been spayed, she has filled out (doesn't help that she's really fluffy), but hasn't put on any weight, and doesn't look at all skinny anymore. Vet has said to try and keep her at that weight as she's perfect.
Roly my other dog also has problems with runny poos, and I found that none of the premium foods helped at all. He was awful on Burns, JWB fish (junior) was ok, but as soon as he went onto the adult I had the same problem. He's now on Chudley's, which whilst isn't the best dog food, it suits his digestion and he is in very good health. Both dogs also have the odd chicken wing every week as well.