Thats probably why.
My 4 dels all slowed down at around 4-5" in my 30g, but once moved to Henwards 1200L tank they had a good growth spurt. I think if you grew some sens out in a 120g+ tank you'd see good growth and they'd get to 12".
IMO a 75g would be ideal, a 12" fish (or a stunted fish that doesn't make it to 12" and dies prematurely) in a 30g tank is cruel IMO...