Since we started keeping records (1914-18 war), average height of the British infantry recruit has been consistently one inch below the average for British males generally. It is a reflection of the poor diet associated with childhood in households at the lower end of the socio-economic spectrum.
In People Of The Abyss, written in 1910, Jack London mentions how the War Office lowered the height requirement 3 or 4 times from 1900 because there were not enough recruits who met the standard.
The industrial slums of Britain and years of poor diet raised stunted unfit young men with all manner of health problems who were unfit for military service.
The mounted soldiers who looked resplendent in their breastplates and feathers were all corn fed strapping country lads.
Then the height restrictions went out the window four years later.