OTL Spain had a population around 9 million c. 1500, but TTL's Spain is OTL Spain and OTL Portugal, which had a combined population closer to 11 million. Over the course of the 16th century ITTL, this population has reached almost 15 million by 1600 (a 36% increase).
A little known fact about the first half of the 16th century is that it was actually a period of large population growth in Europe, with France growing from 15 million to 20 million (a 33% increase), Germany growing from 16 million to 20 million (a 25% increase), and England from 2.1 to 4.15 million (a 97% increase!) between the years 1500 and 1600. Keep in mind that all of these increases occurred in spite of the serious social upheavals and violent rebellions happening in each of these countries during this period. With vast quantities of wealth pouring in from overseas and with the Crown rich enough to invest in irrigation projects, hospitals, orphanages, etc., TTL's Spanish population has benefited greatly and has been saved from some of the disasters and deleterious trends that hampered its population growth IOTL. Likewise, the introduction of American crops such as maize and potatoes have transformed Spanish agriculture and have made subsistence farming much less liable to food shortages and crop failures.
The 19th century population is just a speculation, and is mostly still up in the air although it will probably end up being closer to 20 million. The reasoning for this change (a growth of only 33% over two centuries after a 36% growth in the span of a single century) is that the Iberian Peninsula can only support so many people (even with the introduction of American crops and some small technology advances in agriculture), meaning that the large scale population growth is going to plateau at some point and lead to both famines and increased emigration to the Americas, North Africa, South Africa, or (to a lesser extent) to one of the many other colonies in Africa or Asia.
The reason that the Spanish and Portuguese populations IOTL actually decreased when every other nation in Western Europe was experiencing growth was due to a number of different factors. The 1590s saw a devastating resurgence of the plague (alongside other, albeit less destructive, outbreaks throughout the 16th and 17th centuries), coupled with a complete collapse of the Spanish economy at around the same time - not an economic recession or depression but a collapse, the lower classes had to revert to a currency-less barter system while living in the most silver-rich country in the world. Combined with the continuous drain of emigrants leaving for the New World (admittedly not as massive of a drain on the population for Spain as much as for Portugal) and the large number of people entering into celibate religious lives (there were more than 100,000 religious in Spain in the late 16th century), and of course the many conflicts that enveloped the Iberian Peninsula (the 80 Years War, the 30 Years War, the other Franco-Spanish wars of the 18th century, the War of the Spanish Succession, the Napoleonic Wars, and the Carlist Wars, etc etc etc), you have a veritable litany of reasons for OTL population decline in Spain and Portugal that do not really exist (yet) in TTL's Spain.
In regards to the Dutch remaining under Habsburg rule, I agree that they are unlikely to pursue the same level of colonization that they did IOTL at least for the better part of the 16th and 17th centuries. However, the shipping fleets of the Dutch merchants remain just as massive as IOTL and the central position of the Netherlands in relation to European commerce remains, so some interesting things are still going to happen for the Dutch overseas ITTL.
Thank you. It has been a very instructive explanation. I believed that OTL Spain population in 1500's was much lower, like 6 millions or so.
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