Beside the anglerfish, I can't remember any other animal with that gender size difference that is not a bug.
Now, I'm not sure of the Old Gods being male High Dragons, both the Architect and Corypheus had tainted Dragons at their command, the Red Lyrium is stronger than the common taint so, Corypheus' Dragon had to be stronger than a real Archdemon and it wasn't. We also know that there is another kind of Dragons, the "Great Dragons", my guess is that the OG are male Great Dragons, the only female among them is Razikale and we have not seen her, until see her, I will take it as that she's bigger than the others.
I don't know how relevant this is, but an interesting tidbit of information regarding sexual dimorphism (size/color difference between sexes of the same species) nonetheless - with birds of prey, the female is always larger than the male, generally outweighing him by about 30% which is quite a bit. This, however, is the average ratio; with some species, such as the peregrine falcon or the sparrowhawk (or its North American counterpart, the Cooper's hawk), the female can be about twice the weight (and bulk) of the male. Such distinct sexual dimorphism is due to several reasons: (i) when nesting, everything and anything will try to murder the everloving eff out of the eggs/chicks, including other birds of prey, and seeing as it is the female who spends most of the time at the nest during breeding, she has to be the brawler in the family so as to be able to fight off any potential attacker, no matter how big, and (ii) the size difference between the male and the female means that they also hunt different kinds/species of prey - the male can hunt animals that are too quick and nimble for the bulky female to catch, whereas the female can hunt animals the male is not physically strong enough to bring down, thus ensuring a broader variety of food for the chicks.
tl;dr - it's all about the family and birds of prey are awesome.