As a Dutch historian of short individual stature, I must say that I was rather amused at reading this question for several reasons.

In the first place we must note that The Netherlands (and the Dutch) remained neutral during the First World War. Secondly, it must also be noted that epigenetics and eugenics as you describe them - and as far as I know - was a fringe movement in medicine, natural history, and social practice.

I would contend that the reason that the Dutch went from the shortest in Europe around 1900 to the tallest in Europe around 2000 can be ascribed to several environmental and dietary factors.

1. The Netherlands - much like Spain, Italy, and Norway - was extremely late with its Industrial Revolution. Where Great Britain already around 1750 commenced the associated processes of urbanization and industralisation, it was not until about 1850-1870 that a similar process can be observed in the Netherlands. As such, the associated economic and social processes that provide for healthy diet can also be observed emerging with a similary delay around the 1920s.

2. The First World War also factors into this, as neutrality did not spare the Dutch population from the threat of starvation. The British naval blockade and German unrestricted submarine warfare threatened food security quite seriously, and also limited the average diet to locally growable goods.

3. The organisation of Dutch society from the 1850s to the 1950s was socially segregated to a significant extent, and is often argued to have been 'pillarised'. This in effect meant that protestants, reformed protestants, catholics, socialists, and liberals had their own communities (pillars) in which they intermarried, worked, practiced their faith, and traded. Combined with the existent class divisions, this led to a quite bracketed marriage market leading to a curiously undiversified gene pool. Though this functioned as something of a rule, there were a great many exceptions that eventually paved way for the open society that emerged during the 1960s and can still be observed in the 2010s.

In terms of source material I would like to recommend to you the works of Pieter Geyl; his scholarship is rather conservative, but also very detailed and from the earlier half of the 20th century. Besides this academics Henk te Velde and Piet de Rooy have done extensive research on the political culture and national identity of the Netherlands, and are highly recommended if you want a more structural assessment of the Dutch national psyche and its social trappings.

(EDIT): It must also be noted that the post-Second World War introduction of the 'Volksverzekering' (lit. National insurance) greatly helped mitigate medical issues, and overall allowed for everyone in the country to access effective healthcare services at relatively low cost, thus extending it also to the poorer class brackets.

First of all, you seem to have read some bad history. I could find no reference to the kind of (forced) Darwinism you describe. There certainly is no historical evidence of any kind of eugenics program focusing on eliminating the short, nor is there any kind of evidence that suggests Dutch women somehow having a high partner preference for taller men. On top of that, World War I played no role in this, as The Netherlands was neutral during the Great War and, unlike Belgium, was never invaded by Germany. On top of that, if a large scale war like that would have an impact on the average height of the population, conscription standards would mean that it would be more probable to cause a decrease the average height. As far as I know, most belligerents in the First World War had at least some minimum height requirement for its conscripts (although that height requirement would have been lowered as the war went on and the need for more recruits grew). The changes you describe also have very little to do with changing genetics, especially not anything that can be described as 'drastic'.

Now, it can be extremely difficult to find accurate historical data on the average height of a population, even in western countries, before World War II. In general the aforementioned conscription provides the majority of historical data when it comes to height, which means that the data is derived from non-random samples of the male population. Archaeology can provide some insight in height of a population, but in general archaeological grave sites provide a very small sample, from which it can be hard to derive statistically significant data for a whole population.

As for the actual causes of the Dutch being tall, the primary explaining factor comes from socio-economic changes in its society during the nineteenth and twentieth century. As with most western European countries, a lot of its nineteenth century industrial revolution was at least in part powered by child labour, and urban migration that led to drastic drops in the standards of living and healthcare.. It wasn't until 1874 that the first ban on child labour was passed, for children younger than twelve. In 1875 the first sewage laws were passed, requiring that all urban housing was connected to the city sewer network. Slowly, moving into the final decades of the nineteenth century more and more laws were passed related to basic urban hygiene and standards of living that drastically improved the overall health of the urban population.

In general socio-economic factors are generally believed to be an important contributing factor in the average height of a population. As noted in Nature:

The Netherlands Institute for Social Research to measure the progress of Dutch society using indicators that go beyond mere economic growth. The index incorporates indicators in eight domains: health, sport, social participation (loneliness, volunteering), cultural/leisure activities, housing, mobility, holidays, and possession of assets (9,10). The LSI rose substantially between 1974 and 2008. Between 1990 and 2008, the differences in LSI scores between socioeconomic classes decreased. This was mainly due to a higher rise in the LSI score for lower socioeconomic groups (9). A correlation between height and LSI has not been demonstrated, but one would expect a positive correlation, as with economic growth.

In general, following World War II, under the so-called Rood-Roomse Kabinetten (Cabinets based on parliamentary coalitions between Labour and the major Catholic party), a large social safety net was constructed, including healthcare and government-controlled retirement funds. These provided for a drastic rise in the standards of living. With the post-World War II reconstruction, bolstered by Marshall Aid, The Netherlands experienced steady economic growth, with which came more leisure time for the general population, better diets, better healthcare, all of which, as the Nature article notes, correlate to an increase in height of the population.

Sources:

Yvonne Schönbeck, Henk Talma, Paula van Dommelen, Boudewijn Bakker, Simone E. Buitendijk, Remy A. HiraSing & Stef van Buuren. "The world’s tallest nation has stopped growing taller: The height of Dutch children from 1955 to 2009." Pedriatic Research 73(2013): 371-377.

Pols, Hans. "Eugenics in the Netherlands and the Dutch East Indies." In The Oxford Handbook of the History of Eugenics. Oxford: Oxford University Press, 2010.

Van den Berg, J. Th. J. & J.J. Vis. De eerste honderdvijftig jaar: parlementaire geschiedenis van Nederland, 1796-1946. Amsterdam: Bert Bakker, 2013.