Well, the OTL median annual growth rate for the population of Greece between 1848 and 1907 was about 1,679%. Of course that includes new territories acquired like Ionian islands in 1864 OTL and most of Thessaly (and a small part of Epirus) in 1881 OTL.
If we assume the same growth rate in this TTL (assuming that the territory added will be roughly the same) and start with a population of 1,31 mil., by using this wonderful tool http://www.metamorphosisalpha.com/ias/population.php
we get a result of 3,498 mil. people by 1907. However, as it has been mentioned in the discussion, the greater prosperity of the country, the enhancements in industry and agriculture, plus some fewer immigration, could result at a growth rate of about 2%, with modest calculations. That would mean a population of 4,213 mil people in 1907, which already makes a big difference. If we go for more swift population increase of 2,2% we will get a population of more than 4,7 mil. by 1907!
That increase could have huge impact for the balance of power in the Balkans . In OTL, at the onset of the Balkan wars in 1912, Greece had a population of around 2,7mil, Bulgaria 4,3 mil and Serbia 2,9 mil. As a result of the lower population and the defeat in the Greco-Turkish war of 1897, Greece was not taken very seriously by Bulgaria and Serbia and it was included in the Balkan Alliance mainly because of its Navy. After the Balkan Wars the population figures were Greece 4,4 mil., Bulgaria 4,4 mil and Serbia 4,5 mil.
However, if the Balkan wars occur ITTL (I think they will, but not in the same manner), Greece could be at least equal to Bulgaria and maybe even more populous! Thus, Greece's role in any Balkan alliance would be much more important.
Leaving aside demographic effects from the cholera epidemic during the Crimean war OTL (notable since it was the SOLE time in a century the Greek quarantine measures broke down... at the only time it was the French controlling Piraeus instead of Greek authorities) and assuming the Ionian islands and Thessaly are annexed on schedule TTL population would be about 1,456,000 in 1861 2,134,000 in 1879 and 2,993,000 in 1896. So call it 3.34-3.62 million depending on emigration by 1912. And of course the earlier new territory is liberated the larger the relative population increase it.