This could just be nature protecting the fish. People are always discussing tanks being too small for fish. But in my decades of being in this hobby I have yet to see a fish outgrow a tank. My dad had a yellow tang for 14 years. The yellow was in a 50g tank. He never got more than a 2" body size. Lots of people would jump up and scream that this tang was miserable and unhappy. But 14 years is significantly longer than most fish survive in this hobby.
I have a pair of large Hippos in my 400g tank for 3 years. They are obesely fat, but only about 7" long. If you go to any public Aquarium you will see Hippo Tangs that are over a foot long.
When these fish are kept in our tiny boxes. And even 400g is tiny compared to the amount of area an Achilles can cover in 5 seconds in the wild. Nature protects these fish so they don't outgrow there homes.
And while someone may have an 8" Achilles in a 240 and yours is only 6" in a 400g - Neither one of those fish is the close to the size of some of the SHOW Size Achilles that come in out of the wild at over 12". And I am sure that both of your tangs are fat and healthy.
Dave B