I would probably say it all comes down to better nutrition all round.
I remember my mum remarking that when she was pregnant with us 4 children, mums were told that it was better if the baby was small , to help give them an easier birth!It wasn't the classic "Eating for two" that came in after.
Also during the First and Second World war, there was a lot of poverty in the general public, so nutrition was not that good, often just enough to keep a body going. My own father was born in 1926 and only ever reached 5ft 1 inch in height - his family were all under 5ft 6 inches, whereas my generation men are 5ft 10 inches plus and my son's generation 6ft plus!