I have had 3 south american puffers for 3 or so years, 2 are about 1 and a half inches now but the other 1 is twice as big. Does this mean they are different sexes or have i just got a giant one?
I've read that ~7cm is typical for captive SAPs, half the size of wild ones. Perhaps you are not feeding them enough, so the stronger one from the time of purchase has always had "first dibs" at the snails, cockles and alike offered at meal time? Perhaps the smaller ones have some sort of internal parasite, slowing their growth rate?
Consider me very envious, I really love the idea of a 6+ group of SAPs, but I'm concerned about the clove oil method of sedating them for regular beak clips!