I didn't said that population is the only criteria. But it is one of the factors that need to be considered. Look at the difference between England and New Zealand. The former has a population that is 12 times that of New Zealand. As a result, there are more cricket players at club, first-class and international level. So the player pool is much bigger. The problem with New Zealand is that they have such a small player pool, and in case any of their players get injured, then it gets really difficult to replace them.
New Zealand despite having small population is runner up in two ODI world cups. They are as good as any other team in the world, they just need to perform well on big days.
The numbers of population in New Zealand are very low but yes, their performance is highly appreciable. It is not right that if the population is small, they will not do well, but if the population is large, it is easier to produce cricketers there or it is easier to find good cricketers. If we look at Indian cricket the population is very large, due to which the players have to compete a lot to survive in the team, but in countries where the population is small, they can get a chance in the team only if they do a little bit well. It would be wrong to consider everything only in terms of population. Sometimes small countries also rule in cricket.