I dont think the population numbers are such a casue for discomfiture. The population of mainland Khovaire is just under 16 million. Medieval Europe had by 1000 maybe 30 -40 million. However, I doot think you can correlate Earth and Eberron on a one to one basis.There are a few points Id keep in mind:
1. The "magitech" level of being roughly 17/18th century isnt necesarilly so. I see Eberron as very much a medieval place, with Final Fantasy esque tech added on. Again just because their are fantasy version of zepplins and trains doesnt make it Georgian, or for that matter, Edwardian England's correlary. Many things that make popualtion growth possible- sophsitcated methods of agriculture, stable trade networks, countries trading on somewhat modern system of money and credit dont exist in alot of Khovaire.
2. There are large regions that are unarable or depopulated or cant really support large populations. I mean you need to have places to grow food put horses to graze etc, magic or not. The Demon Wastes, Mournland, Talenta Plains and Shadow Marches dont look like prime agricultural areas.
The druids of the Eldeen Reaches wont like if you start chopping down trees to put up subdivisions, so cultural brakes exist on population growth too.
3. War would probably have a big effect on population level. Cyrene was destroyed in a magic A bomb type catastrophe. War brings disease famine and all other kinds of suffering. Just becasue you have magic healers doesnt mean its applied equally across the world. Look at our own world - how many people die in Third World nations of afflictions easily cured in the West?
4. Demographics of a fantasy world is different than ours. Many nations are dominated by humanoid or nonhuman races. Elves, orcs, goblins, gnolls, dwarves. They dont necessarily reproduce with the same frequency or reliablity of our world. And many of those nations have continuing ongoing fighting and fueding which would bring down the population a little.
BTW, I would not hold too much faith in the numbers given that Xerxes invaded ancient Greece with a million men. Ancient and medieval sources loved to boost the numbers in their histories, so while it was formidable it may not have been a million.
Just my 2 cents.