The country’s official name is: UNITED PROVINCES OF SOUTH AMERICA, UPSA (Meridian Spanish: PROVINCIAS UNIDAS DE SUDAMERICA, PUS).
Population: ca. 39 million. Note this figure is uncertain due to some sources citing citizen population and some citing resident population: a significant number of residents of the UPSA were non-citizens either from semi-autonomous native states and reservations or from other Hermandad states, either seasonal workers or permanent settlers. It could be as little as 37 or as much as 46 million depending on one’s interpretation.
Estimated OTL Population: 13,548,000 (assuming Paraguay never had the 50% population reduction from the Paraguayan War)
LTTW OTL Difference: 25,452,000 surplus
The country’s official name is: EMPIRE OF NORTH AMERICA (ENA).
Population: 54 million (based on estimated growth since 1890 census).
Estimated OTL Population: 75,496,000
LTTW OTL Difference: 21,496,000 deficit
It is very noticeable how it seems UPSA gained as much as ENA lost in terms of population. We know the reasons for this difference (ENA being far more exclusive for much longer, combined with UPSA's political and economical development), but I never thought it would be this different. Quite interesting how these numbers were arrived at, and it shows that UPSA does have the population foundation to be a great power in this world.
The country’s official name is: RUSSIAN EMPIRE or EMPIRE OF ALL RUSSIAS.
Population: 155 million (approx. – Imperial census does not currently extend to all the reaches of the Empire)
Estimated OTL Population: 134,597,000 (Not including Lithuania or Finland which added would result in 147,603,000)
LTTW OTL Difference: 7,397,000~20,403,000 surplus
Given that Yapon's population would have been massively reduced compared to OTL from all the plunder, resistance, and massacres, this gain shows just how more developed Russia is with the earlier access to railroads and various geopolitical offenses.
The country’s official name is: EMPIRE OF THE GREAT FENG (DA FENG), commonly called FENG CHINA or occasionally SOUTH CHINA.
Population: 290,000,000 (estimate)
Estimated OTL Population: 303,171,000
LTTW OTL Difference: 13,171,000 deficit
The 19th century has been unkind to China, since Feng has less people than OTL which had the Taiping Rebellion offing at least a minimum of 20 million. I suppose a more general civil war combined with extra deaths from floods really compounded.
I'll do the European countries another time, since they require a lot more research than the above.