In all the analysis (and formula) for predicting medal count, I don't see any mention of climate or geography, especially when it comes to outdoor sports. This obviously has a huge impact on the medal table for the Winter Games, with countries like Norway and Switzerland punching way above their weight; why not in the Summer Games too?
Australia, South Africa and New Zealand have always been renowned "sporting nations", and I suspect it has a lot to do with the fact that they have warm climates and lots of sunshine, so people tend to spend more time outdoors; whereas many countries in Europe have to contend with lots of rain.
They also have lots of warm-water coastline, which is likely to help in sports like swimming, sailing and surfing. Indeed, simply being big (in area) is likely to make a difference, since a large country is likely to contain more varied climates - the USA and China being obvious beneficiaries on that score, since they range from Northern Continental to sub-tropical.