You call it a myth.. but if I am not mistaken most fish exude a hormone that inhibits growth? which is partially why water changes are so important? and in 20yrs of keeping fish, I've never seen an arrow in a tank that size from a juvie not be stunted in a tank of that size. personally I'm leaning towards the belief thay bought the fish that size and put it into that size of a tank. and it hasn't been in there long. there seems to be no lack of healthy muscle, the fish obviousely has little to no room for exercise. it's a toned "healthy" looking fish. I'm not saying it's not possible... But I highly doubt this fish grew-up in that tank. and I highly doubt the fish has been in that tank long.
This person is selling the fish, not the tank.
What type of arrow is it? I'm not farmilular with asian arrows since they are illegal in the states. but if this is the case with this fish.. That explaines alot...
It is possible... But... I smell something fishy with that picture.