Breed standard for adults is 12-18lbs in Europe and 13-18lbs in the US. Some dogs will naturally fall under or over that weight for a range of reasons and may still be fit (a dog should not weigh within the breed standard; it should weigh whatever is a healthy *lean* weight for its build
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Many cavaliers are over breed standard, especially males -- it is a lot more common for non-show breeder-bred dogs in particular to be over the standard. For comparison -- easily fewer than 20% of the cavaliers I got into rescue over 7 years were within breed standard.
I wouldn't classify any complete dog food that meets standards as "bad". I know plenty of dogs fed nothing but who have long outlived dogs fed the fussiest premium brands. There's a lot of nonsense and politics around dog food (more so than government politics, believe me! :lol
. Supermarket foods are generally fairly mediocre (the ingredients won;t be the best quality, to keep prices low) and tend to have preservatives and colouring to make them look like they have little coloured veggies etc.
For a small subsection of dogs, such things may cause allergy issues but this is a very small subset of all dogs.
It's funny but the same people who are food dictators tend to be people who happily eat all sorts of human junk food themselves, going on my own encounters... :lol:
I'd not get too worried or worked up about dog foods -- every single one has its supporters and detractors. My advice is to find a few you like that fit your budget and then rotate types -- when one bag is totally finished, try something else or another flavour. Variety for dogs is good. Personally I wouldn't bother feeding puppy food at all. Dogs in the wild would never get anything different from what adult canids eat once weaned and many good breeders advise against feeding puppy foods as being too over-packed with nutrients, causing too fast growth (especially true with giant breeds).