No, many of them are just very good at 'power growing' their betta. If you watch the breeder section of the forum, you'll see that one person's two week old bettas will be the same size as one person's one month old bettas. It all has to do with water quality and food. The fry give off hormones that cause the other fry to stay small, so if you don't keep the water clean enough, they stay TINY! Thai breeders have a lot of space and time and the added benefit of being where bettas originated, so it's easier for them.
I think the problem with them going to stores when they are so small is that stores don't take good care of them--so after the stress of shipping(which alone can kill even a healthy betta) they are put in a place with cold water and cold air and the water is allowed to get filthy!
The tiny tiny ones at stores look like they are even younger than the 3.5 month old guy there. My guy was 3.5 months old and was still tiny tiny. Not as small as these guys at the store though!