I've seen estimates on Wikipedia that put the Ottoman population in Anatolia and Syria even lower in the early 20th century than it was in the Middle Ages.
Are these accurate or are they pulling them out of no-where? And if they're actually accurate - how or why was the Ottoman population so low? I've seen references to repeated cholera epidemics in the 19th century, but why would cholera suddenly have started killing so many people in the 19th century and not earlier? Or why were the Ottomans so severely struck by cholera?
I’m more familiar with Syria and so can speak to that. The population numbers you’re looking at I imagine come from 19th century Ottoman censuses. These were conducted for tax purposes, as among other things the Ottomans levied a poll tax (a tax paid based on the number of residents) which was collected locally and paid by the village as a whole (I’m massively over-simplifying, the Ottoman taxation system was high localized and non-uniform, and Ottoman administration in general underwent significant reforms in the 19th century.)
The result was that villages had a significant incentive to under-count their populations. As a result, there is good reason to suspect that 19th century Ottoman census figures are significantly lower than the actual population numbers.
That being said, there certainly were areas where the population figures in the 19th century seem to be much lower than the Middle Ages. Jerusalem, for example, appears to have dwindled from a small city to a large town of only a few thousand people.
To my knowledge this has less to do with any kind of disaster like cholera than with general regional population dynamics—Jerusalem just wasn’t a major center of trade or government by that point.