One of our LFS (sadly gone now) had a monster floor tank that showed this principle rather graphically.
The bala sharks that had been grown out in a properly sized tank kept a nice streamlined body shape. Beautiful, fast swimmers, good eaters, just gorgeous. And then there were the ones that came from customers desperately seeking a new home for the bala they had kept in smaller tanks. They had shortened, heavy bodies, thick thru the abdomen, with visibly bowed spines. Alive? Yeah. Healthy? Well, they swam, ate and pooped. Personally, I have higher standards for my fish than mere survival.