Freshwater lakes accumulate their volume in a manner analogous to the way that an underground reservoir accumulates oil. Over geologic time, water drifts into a basin at various rates and over a range in collecting regions. In the context of oil reservoirs, I have talked about this behavior before and the Maximum Entropy prediction of the size distribution leads to the following expression:
P(Size) = 1/(1+Median/Size)
Surveys of lake size show the same reciprocal power law dependence, with the exponent usually appearing arbitrarily close to one. In Figure 1 below, the data plotted on a ranked plot clearly shows this dependence over several orders of magnitude.
Figure 1: Northern Quebec lakes [1]
More revealing, in Figure 2 we can observe the bend in the curve that limits the number of small lakes in exact accordance to the equation. The agreement with such a simple model suggests that a universal behavior links the statistics between environmental phenomena as seemingly distinct as those of lakes and oil reservoirs. 
Figure 2: Amazon lakes [2]
This provides other intuitive clues to how to think about reservoir sizing. Consider the fact that very few freshwater lakes reach gigantic portions, the Great Lakes serving as a prime example. Similarly, the rare occurrence of “super-giant” reservoirs follow from the same principles. We clearly won’t find any new huge freshwater lakes, while the future occurrence of super-giant oil reservoirs remains very doubtful just from the statistics of oil reservoirs found so far. Finding substantial numbers of super-giant reservoirs would result in deviations from the size distribution plot, making it very unlikely.
References
[1] K&C Science Report – Phase 1 Global Lake Census
[2] Estimation of the fractal dimension of terrain from Lake Size Distributions