It is also a good idea to look to the parents if possible and ask the seller how old the are.
It is really important that they eyes are in the right comparison with the body size.
My advise for discus is not to buy them online but have a look at a special fish store that also have knownledge about discus fish.
Maybe you can find a fish store where they can deliver directly from a good discus breeder in Asia or Europe.
There are some good US breeders out there, now.
I think those discus look a bit underfed. Their foreheads should be taller, they should be more rounded overall. It really depends on their hatch date, if they aren’t fed heavily in their early lives some discus never attain a good size, aka become stunted.
It could also be that some of those fish are significantly older than the others, but are the same size. If they are from two or more different spawns that were months apart then the younger ones will grow much faster and the older ones will seem never to grow. I have heard some stories about shady practices where people conceal those facts from consumers to offload unwanted stock. In fact, I am somewhat certain it has happened to me already. Caveat emptor is never more apt than when buying fish or used cars.
I looked through the “stunted” thread, but it seems to focus on larger discus, these are only 2-3”. 
