First I would I would ask where your figures are coming from. Outside of western Europe, population censuses were entirely unreliable for virtually all of the 19th and in some cases into the 20th centuries. But, in answer to your question the starting population figures in 1800 were even lower. To quote from Roger Owen's The Middle East in the World Economy:

With the exception of Egypt (in 1897 and 1907) no reliable census was taken in any Middle Eastern country before 1914. Instead, such estimates as exist were based on badly kept tax· registers, on the guesses of local residents, or on such simple expedients as an attempt to count the number of houses in a particular town or the numbers of men attending Friday prayers in the principal mosque.l And if this was the best that could be done for the towns how much more imprecise must have been the occasional guess as to the population of the countryside? In these circumstances the best that can be done is to suggest that, subject to a very wide margin of error, Anatolia and the Arab provinces of the Empire may have contained some 11 to 12 million inhabitants in 1800, of whom roughly 6.5 million may have lived in Anatolia itself (excluding Istanbul), 3.85 million in Egypt, and between 1 and 1.5 million each in Iraq and Greater Syria - including perhaps 300,000 in Mount Lebanon and the immediate environs.

As to why that was, going to back to Owen:

For the rest. the best that can be said with safety is that in the Middle East, as in any other pre-industrial society. the level of population was constrained by three inter-related influences: the high mortality rates and low life expectancy. debilitating disease, and sudden major fluctuations due to wars. famines and epidemics. Contemporary estimates throw light on only the last two of these features. As far as epidemics were concerned, Panzac's work on eighteenth· century Izmir suggests that in particularly bad attacks of plague a Middle Eastern city could lose as much as a third of its population.11 Debilitating diseases were also rampant, as can be be seen from the French Expedition's study of ophthalmia in Cairo which suggested that one in every three Egyptians was one-eyed and one in ten totally blind.12 For mortality and life expectancy, the only guide is provided by the first vital statistics established for Egypt and Palestine during the 1920s. These give death rates of 26.2 per 1000 for the former and 28 per 1000 for the Muslim inhabitants of the latter. as compared with 12.3 per 1000 for England during the same period.13 As for life expectancy, the first Egyptian natural life tables produced in the 1930s estimated that the average male Egyptian could expect to live only until the age of thirty-one, roughly the same span as that of a late thirteenth-century Englishman.

As he concludes:

During the nineteenth century the population of that part of the Middle East covered by this survey [i.e. including Egypt] increased by roughly 300 per cent, from somewhere between 11 to 12 million to approximately 32 to 33 million.