I agree it's complicated, but I honestly don't see much evidence that the "will" of many large countries is directed towards many concerns that don't, directly or indirectly, affect themselves. That is, if a country is concerned with humanitarian concerns somewhere else, it is largely because that other place has sone kind of economic, security, or other relationship with them.
This is much more of a factor, I believe, in a large country. A smaller more cohesive nation might be able to make a more humanitarian stand in a united manner ... perhaps. This is purely a matter of scale and momentum - a jogger can manoeuvre more adeptly than a 747.
It depends on how deeply you want to delve into the humanitarian aspect though. The US, UK, Australia, Canada, and many others are very giving in the humanitarian aide dept. Some of it official gov't aide, but much also through private charities. In regards to gov't aide - yeah, that may be self-serving in the effect that with food and supplies a given nation might be made more stable and in turn, making us safer. Maybe we can even get some financial gain out of a country that we stabilize...etc etc. But on the flip side, what does your average person get out of giving to a private charity? The amount of private aide coming from your country, mine and many others is huge - I seriously doubt that all of those who support private charities are doing it in a self-serving mindset.