Myanmar (known as Burma until 1989) in 1840 (4m) in 1910 (10m) in 1954 (20m) in 1975 (30m) in 1989 (40m) in 2003 (50m) 2019 (54m)

Note most people continue to use both names interchangeably without much fuss. Sometimes Burma is just an easier word to pronounce. Both words have the same origin, linguistically. Myanma is the literary, written name of the country and Bama is more colloquial.

Captured Thailand's capital city in 1767, burning the palace and the city, but driven out by Chinese.

First Anglo-Burmese War then came in 1824-1826, and finished with Burma losing territory she had earlier gained in Assam, India.
Second Anglo-Burmese War 1852-1853, finished with Britain annexing Burma but with no treaty and which occurred in the midst of Burmese-Siamese War 1849-1855 with Siam invading Burma but being driven back.
Finally Third Anglo-Burmese War with Siam neutral (Nov 1885), finished with Burma fully under British rule until independence in 1948.

Mandalay in 1870 100,000

Yangon (known as Rangoon until 1989) 10,000 in 1824, 100,000 in 1872, 248,000 in 1901, 500,000 in 1941, 1.5m in 1960, 3m in 1990, 7m today.
Largest city. Fishing village that was captured by British in 1824 and became the capital city after 1852.



Thailand ("Free man") in 1800 (4m) in 1850 (5m) in 1880 (6m), 1910 (8m), 1927 (10m), 1939 (15m), 1951 (20m), 1964 (30m), 1974 (40m), 1984 (50m), 1996 (60m), 2018 (70m)

Bangkok (Krung Thep) in 1880 (600,000), 1950 (1m), 1972 (3m), 1980 (5m), 1990 (7m) 2018 (14m)

Country known as Siam by outsiders. In 1939, army officer and Prime Minister Phibun insisted the name be changed to Thailand. He collaborated with Japan until opposition to Japan in Thailand forced him to resign in August 1944. He returned to power with military backing in 1948 and the army ran Thailand with support from the US.
 
Thailand was ruled by Cambodia (the Khmer Empire) from 800-1350.
Ayutthaya (Thai Kingdom) became the capital city 1350 - 1565, but was temporarily captured by Burma 1565-1584.
Monarchy then regained control, and the capital city grew in size until defeated by Myanmar in wars 1759-1767. In 1767 the city and palace were burnt to the ground.

In 1768 King Taksin the Great relocated the capital to Thonburi, part of modern Bangkok (Krung Thep). He was overthrown and executed in 1782. It was said he had become insane, demanding worship as a Buddha, flogging and enslaving hundreds of monks who refused to do so. Others said he was simply betrayed.
He was followed by Rama I (1782-1809) who was first king of the current reigning Chakri dynasty, who had served King Taksin in his wars against the Burmese. He was followed by his son Rama II (1809-1824), his son Rama III (1824-1851), his half-brother Mongkut or Rama IV (1851-1868) who was that king (born 1804) in "Anna Leonowens (1831-1915) and King of Siam", his son Chulalongkorn or Rama V (1868-1910), his son Rama VI (1910-1925), his brother Rama VII (1925-1935) who abdicated following succession complaints, his nephew Rama VIII (1935-1946) who was found shot in his bed at his death, his brother Rama IX (1946-2016), finally his son and current king Rama X born in 1952 (2016-today).

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