I believe it was written in the 60s. The older brother has stunted growth because of eating highly processed food, while his younger sister grows tall and healthy. They encounter fresh air, grass, and a stream full of fish, I think. I also believe it was part of an anthology of short sci-fi stories.
This sounds like Crash Landing on Iduna, by Arthur Tofte.
While not a short story, it's a fairly short book. Synopsis: Lars and Iduna Evenson take their family, and leave an overpopulated and regulated Earth to try life on a wild new planet. Things go wrong, they crash. Mother is killed, father is badly wounded, leaving the four children - Peder, Inga, Bretta, and Sven - to figure out how to survive. They have to learn to kill things to eat - the above-mentioned fish are the first things they manage - and encounter the local sapient species - the otter-like Singing People. There are some adventures along the way, and a skirmish with an 'evil' tribe of the aliens. It ends with the older brother - who is the narrator - bitterly commenting on the fact that his youngest siblings were allowed to grow to their full potential, and not be stunted by the design of the government-issued food as were the rest of them who had grown up eating it. "Inga and I were only a little over a meter tall...a matter of less than four feet on the old scale!"
My copy is from the Laser Books series, and features a Kelly Freas cover in green, with a kind of monkey-like Singer carting a blonde woman around behind the obligatory-floating-face-in-the-corner.