With Japan being so much smaller than China how were they still able to successfully combat China in the early 20th century?

My answer on Quora.

At the risk of using another cliche - ‘Size does not necessarily matter.’

The UK, a small island nation, created and built the largest Empire that the planet has ever witnessed with holdings on six different countinents. France, Spain, Portugal and the Netherlands had their own success in this regard. In the Middle Ages the tiny city state of Venice was a regional power.

What seems to be the key driver in the game of power politics is a combination of aggressive leadership (will to power), economic prowess, geographical positioning, technological innovation and a well organized military.

The Japanese had the edge over China in all five of these aspects, thereby negating the apparent population and size advantages that should have given China the edge.

In the 19th century Japan took it upon themselves to modernize and they did so in rapid fashion. This gave them a tremendous head start over their Asian rivals and it wasn’t long afterward that Japan as a military entity could rival some of the European powers.

The Japanese victory in the Russo-Japanese War (1904–1905), where they completed trounced the Tsarist forces was indicative of their progress. This followed on from the Sino-Japanese War (1895) where Japan’s victory against China guaranteed them control over the Korean Peninsula.

Japan’s success on the military front was consistent with the development of a strong land and naval force. However the political revolution that drove this advancement traces back to the Meiji Period (1868–1912) . During this time Japan, initially under the Emperor Meiji. broke from the Feudalistic structure of the Tokugawa shogunate and adopted a new ethos outlined by the Five Charter Oath (1868).

The provisions of the oath were -

1. Establishment of deliberative assemblies;
2. Involvement of all classes in carrying out state affairs;
3. Revocation of sumptuary laws and class restrictions on employment;
4. Replacement of "evil customs" with the "just laws of nature"; and
5. An international search for knowledge to strengthen the foundations of imperial rule.

While the Empire’s position as the ultimate Head of State was ensured, democracy was introduced and the Old Han system (still very much alive in Qing Dynasty China) was abolished. Japan came of age.

Source: Historical jury still out on Japan's Meiji Restoration | The Japan Times

The transition wasn’t as smooth as some like to portray but the Army would emerge as a significant power within the nation taking over the role from the earlier warrior classes. Since Japan is resource poor, territorial expansion was very much on their agenda.

Source: by-yoshitoshi-4 - Meiji Restoration Pictures - Samurai and Bushido - HISTORY.com

China in contrast lumbered for some time with the weak and corrupt Qing dynasty and even after the end of the last dynasty in 1911 (Xinhai Revolution) still struggled internally with disunity and Civil War, up until the Communist takeover in 1949.

Sources:

De Bary, William; and Arthur Tiedemann (eds.) (2005) [1958]. Sources of Japanese Tradition, Vol. II: 1600 to 2000 (2nd ed.). New York: Columbia

Breen, John, "The Imperial Oath of April 1868: ritual, power and politics in Restoration Japan," Monumenta Nipponica, 51, 4