It is possible that this is a very very large example of the breed, as others note.
However to be honest, and I am sorry to say this, I would wonder about whether you were sold a puppy that is only 12 weeks old. That sounds to me more like a dog that might be at least 6 months old. I have seen this happen before with certain types of breeders, who have older dogs they failed to sell and offer them as far younger than they truly are. Otherwise, I would wonder whether this is a cavalier, or perhaps a cross of some sort to a larger breed. Most cavalier puppies would not be that size and weight til at least around 6 months. As others note, the weight is fine for the height of the dog, but normally that would be a far older dog. I have two mature, adult dogs that size and weight (actually, a little shorter).
What does your vet say about this pup's size and age? Can you post a couple of pictures? Many here would likely be able to tell you whether it is likely that your puppy is 12 weeks old. The pic you posted on your intro thread to me does look like an older puppy than 12 weeks -- but also his muzzle is longish which might suggest there's some cocker or similar larger spaniel in his background, accounting for his size. His nose is still not fully black and assuming this isn't a fault (eg the pigmentation may remain mixed pink and black) that could also indicate a younger puppy that is just very oversized, as most noses fill in by 6 months.
Much depends on the quality and honesty of your breeder. Ideally s/he is not a backyard breeder or puppy farmer using some of the meaningless registration services for this puppy -- in other words, as a very basic starting point (but still no real indication of the quality of the breeder), the breeder should be registering puppies either with the AKC or the CKCSC in the US, and no other registry. Then you would have at least, evidence that the breeder claimed this litter was born on a specific date. They could still make up this information sadly -- as there is no way except DNA testing to verify their claims -- but too few breeders do this voluntarily and most won't, I think, til it is mandatory.
If the puppy is registered AKC or CKCSC and your vet feels there is no way this is a 12 week old puppy, then you can file a complaint against the breeder.
If the breeder has neither of these registry affiliations (AKC or CKCSC), then unfortunately there's no way of knowing if you have a cavalier, or a cross, or the family history as many such breeders are not likely to be honest about background and other 'pet' registries used widely by backyard breeders and puppy mills, will register absolutely anything. No reputable breeder uses them.
All that said -- there are
definitely some large cavaliers out there
-- I've had one as large as 35lbs adult size in rescue -- but that large is really rare and probably means somewhere in the background of that line their was a larger breed. The origin of the breed as it was reconstructed in the early to mid part of last century is a bit murky and some lines it is claimed included larger spaniel breeds and larger sized dogs may be throwing back to those. It is all a subject of much discussion amongst cavalier fans.
There are many previous threads here with input from some knowledgeable people on the origins of the breed.
For a cavalier to be that large at that age would suggest you will end up with a dog weighing up around 30+ lbs or so -- most cavalier puppies that age would be perhaps around half that weight, perhaps a little more but not much. Most cavaliers won;t have reached that height -- which is the full standard height for an adult -- for many more months, suggesting you will also have a much taller dog (more cocker or small springer spaniel size).
What would most concern me here is that the breeder did not fully discuss the very large size of this puppy with you, if he is truly 1) 12 weeks old and/or 2) a cavalier -- this would be a very non-standard dog and any cavalier breeder should have been aware of this and had some sort of explanation (just as one also should for a very undersized puppy).
Of course what matters at core here, is that you love your dog and size should not matter. Breed standards matter primarily for showing dogs, not for our pets.
But if there's the possibility that a breeder has been deceptive, I would want to take action against that (though there are so few protections for puppy buyers that there's likely little that can be done even if they did sell an older dog as a younger one).