Sample size affects variability. Small countries appear at both the top and the bottom of any given league table because more populous nations begin to approach the mean.
It's the same reason that the best and worst schools are small schools; the best and worst health outcomes are in rural areas and so on.
Nah. Small northern European countries outperform the mean consistently. Even the baltics are doing better than Poland. Not everything is random variation. Big countries (Pakistan: 186m, Congo: 70-something) can be pretty fucked up too. Your assumption is that the underlying natural value is independent of country size, mine is that it isn't.
It's the same reason that the best and worst schools are small schools; the best and worst health outcomes are in rural areas and so on.