It's unlikely that you ended up with a miniature bearded dragon. More likely your dragon has parasites, which are stunting its growth.
Rankin's dragons (sometimes referred to as miniature bearded dragons) are less popular in the pet industry, so it's unlikely that you got one by accident. Although Rankin's dragons and Inland Bearded Dragons can interbreed, and it's possible that someone could have introduced a Rankin's dragon into their gene pool, I would probably discount it entirely unless you know you got your dragon from a breeder who keeps both species intermingled.
The easiest way that you can tell if your dragon is a true Rankin's dragon, is that true Rankin's dragons don't have beards. You can tell if they have a beard by showing them a mirror, so that they think there is a rival bearded dragon next to them. Or by watching them while they're basking, as they will sometimes stretch their beard.
Other ways they differ are going to be less noticeable unless you have a direct comparison. Compared to a bearded dragon they have a more rounded head, and they are considered more social than inland bearded dragons, allowing or multiple females to be kept together (males should still be separated). They're also a bit more skittish than bearded dragons, but my opinion is that it's because they're not popularly kept as pets.
Parasites on the other hand, can stunt a bearded dragon's growth. Bringing it to almost a standstill when they're young.
Actually, it's considered normal for bearded dragons to have low levels of Coccidia in them. They're a single-cell parasite that live in their stomachs. They don't cause any harm unless they reproduce too much, then they'll cause symptoms like diarrhea and dehydration. They pass through feces so keeping the cage clean will prevent them from spreading.
The parasite that I believe would be causing your bearded dragon's stunted growth is pinworms. Pinworms like in the digestive tract and steal nutrients. Causing symptoms like a lack of appetite, weight loss, dehydration, and changes in stool (either diarrhea or lack of it altogether).
I would advise a trip to a vet with experience with reptiles, along with a fecal sample, so that they can diagnose the cause for sure, and provide you with the correct dosage of anti-parasitic medicine. Along with some high-nutrition food to help supplement his diet for the next few days to get his strength back.
The only advice I can give for avoiding parasites like pinworms in the future is to keep his cage clean, and avoid contact with other reptiles that might be infected. I've had problems with buying crickets from local pet stores, so I've taken to ordering from stores online. It ends up being cheaper than from the store anyways.
Other things to keep an eye on that might be causing your bearded dragon to not be growing are the heat and light parameters of their enclosure. As reptiles they need to have the proper temperatures of 80 degrees fahrenheit, with a 100 degree fahrenheit basking area. And a UVB light to help them digest their food. UVB lights will also stop producing UVB rays before the light actually goes out. So be sure to replace them on a schedule of about every 6 months.
