Kings are said to be created in Malaysia - a cross between splendens and betta raja (raja means king)...
Unfortunately, all my "Betta appreciation" experience is photographic. Having said that, anyone would notice that Betta raja has a unique appearance. None of the Giants or "Kings" that I have seen bear even a remote resemblance.
Petco calls their larger-than-regular Betta by their marketing term "King." Do you think for a minute anyone at Petco knows what a Bett raja is? The confusion is only in the minds of the more sophisticated and educated Betta fancier.
From all the pictures I've seen they all look the same.....In 4 months Kings and Giants should reach at least 5cm/2" body only.
From discussions on other forums, plus the spawn log of a current member, I am drawn to the conclusion that all Betta larger than 'regular-sized' result from efforts to breed larger fish. Like 'regular' Betta, the size varies depending on the quality and size of the parents and the success of the breeder.
Some Aquabid "Giants" are photographed and sold at 2". Some Petco "Kings" are bigger than that. They appear to be young fish, and probably are, for the reasons that Injo describes. I think the size of a Giant varies within a certain range, perhaps even overlapping that of larger 'regular' Betta. Perhaps size is less a valid indicator than genetics (is there a Giant gene?) or the intent of their breeder.
Though whole sale is cheaper, but I don't believe that you guys are being ripped off. Chain stores must pay for shipping and handling too (king and giant shipping = twice regular), plus daily care (supposed to anyway) until they're sold. Calculate all that into cost, it's fairly expensive.
I have always maintained that Betta, regardless of size, color or type, are incredibly cheap, considering the cost of breeding and, especially, the cost incurred by the retailer to maintain and display individual fish. In fact, they are entirely too
cheap, which is one reason for the abuse and poor husbandry that these fish endure.