Jarv was underweight on Burns (and disliked it as well) - some dog's don't thrive on it, I remember Nic's Alfie being the same.
I changed to Arden Grange, fed a normal amount and not only did he really like it, his coat got better and he maintained a decent weight.
If you feed over a recommended amount your dog may suffer from diarrhea and you will find in general they poo a lot more - getting rid of the stuff they don't need and can't digest.