I mean are they similar or did you just pick a few and claim they are? Germany is nearly twice the population of Spain despite being the smaller nation by far, put that way it's not very similar. It also doesn't account for the nations that don't fall within this range which is literally every other western European nation. Holland, Belgium, Switzerland, Portugal, Luxemburg, Denmark, Norway, Sweden, Iceland (technically Germany too), Finland Monacco, Greece and so on depending on where you want to draw the line for "Western Europe" these days. Anyway, most Western European nations do not fall within this range, only 4 do, there are 23 or so that don't. Even if we look beyond this, these nations are not very similar in demographic terms.

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My best answer is, there is not reason for these four nations falling in that range other than you arbitrarily setting the range to include these nations.

Official 2018 figures for Western Europe (rounded to nearest million)

Germany - 82m (236 people per square km)

UK - 67m (275 people per square km)

France - 65m (119 people per square km)

Italy - 59m (202 people per square km)

Spain - 46m (93 people per square km)

Netherlands - 17m (507 people per square km)

Belgium - 11m (380 people per square km)

Portugal - 10m (112 people per square km)

Sweden - 10m (24 people per square km)

Switzerland - 8m (216 people per square km)

Denmark - 6m (136 people per square km)

Norway - 5m (15 people per square km)

Ireland - 5m (70 people per square km)

So what we can see here is:

a) Population size varies a lot by country.

b) If you compare Britain, Germany and Italy, you can see that these countries' populations isn't far off, because the size of the countries isn't far off. Similar size countries you'd assume would have similar populations. Except... not. France has slightly more people than Italy, and slightly less than the UK, yet it is far, far bigger. Spain and Portugal also have much smaller population densities, similar to France, with Denmark slightly more dense. Ireland has less. Sweden and Norway have the least population density - the freezing cold and, again, being on the edge of Europe, are pretty obvious reasons why. The Netherlands and Belgium, on the other hand, have much higher population densities.

So, we can see that population sizes in Western Europe vary massively, but that this is not purely due to the country's size. Western European populations are not at all spread evenly. Each country has geographical, political, cultural (birth rates+migration) and historical reasons for its population.

For example, Ireland's population is low for its size due to a history of poverty, being on the edge of trade routes until the discovery of America, political oppression by the British, the effects of the 19th century potato famine, and a history of emigration (to Britain and the USA).

Unfortunately, as each country's reasons are so particular, you'd need really a separate question for each to get good answers: why is France's population density so low compared to the UK and Germany, why is the Netherlands' population density so high, etc.