Stephen M. Walt in Foreign Policy:
Here's an IR theory puzzle: Why do some seemingly powerful states exert relatively little influence on world politics, while other states with more modest capabilities cast a bigger shadow than one would expect? Although there is no consensus on how national power should be defined or measured, most IR scholars would probably agree that there is a substantial but not perfect correlation between national power and international influence. Indeed, one could imagine a simple regression, with “power” on the X-axis and “influence” on the Y-axis, and a diagonal line bisecting that space. I'd expect most states to array themselves pretty close to that line: as their power increased (measured in terms of GDP, population, military capability, resource endowments, etc.) one would expect to see a corresponding increase in their global influence.
But what about the outliers — either the “overachievers” who swing a bigger bat than one would expect or the “underachievers” who wield less influence than their overall capabilities might provide? Here's my personal, decidedly un-scientific top five list in each category, followed by some thoughts on what might explain why some states punch above their weight and some potentially major powers cast a comparatively small shadow.
“OVER-ACHIEVERS” (in no particular order)
With a population of only 9 million, one wouldn’t expect Sweden to cast much of a shadow, despite its advanced industrial economy. Yet for its size and population, Sweden has been a significant international player. Its welfare state and other social policies have been widely-studied and a model for others, and diplomats such as Dag Hammarskjold, Folke Bernadotte, and Olof Palme were all important international voices. Sweden still devotes a higher percentage of its GDP to foreign aid than any other country, and institutions such as the Stockholm International Peace Research Institute have amplified Sweden's visibility on major issues of arms control and disarmament. Awarding the Nobel Prizes probably doesn't hurt either.
More here. [Thanks to Kris Kotarski.]