While landmass helps to determine the population size that it can support, there are a number of other factors that come into play. For example, populations tend to grow in areas that have a climate that isn't harsh, access to resources like fresh water and arable land, and ports that can be used for trade.
Looking at this map, which shows how the world's population is dispersed, you can see that these factors are important. Much of the population is concentrated along the coasts of continents and sparsely populated in regions that experience arctic temperatures. Nobody enjoys being cold!