Sweden: Pop. 9,182,927
Thailand: Pop 62,828,706
Erm.. thats todays population. Which is somewhat irrelevent.
Using
http://www.populstat.info/ which may or may not be accurate,
In 1870~ Thailand had about 6~ million.
In 1900 Thailand had a population of about 6.3 million.
By 1920 it was about 9 million.
By 1940 it was 15 million.
Now admittedly, Sweden had a population of around 4.3 million in 1870, 5~ million in 1900, 6 Million in 1920 and about 6.3 million in 1940.
So if there was near uninterrupted reform then eventually Thailand would likely have quite a bit more clout.
But to compare it with Japan.
In 1872 Japan had a population of 33~ million.
In 1900 Japan had a population of 44~ million.
In 1920 Japan had a population of 56~ million.
In 1940 Japan had a population of 72~ million.
Thailand could potentially have reformed herself and industrialized, but she isn't going to be a second Japan. She doesn't have the population which directly means to her not having similar military power or economic strength.