The Long Tail, one of the best selling books written by Chris Anderson, describes the importance of items with limited popularity and rotation, that could not find their place in standard supermarket shelves.
The cost of inventory for Amazon for instance, has made many “niche” books accessible.
The new Top Level Domains are typical of that distribution model, where it does take some effort for a registrar to sell a new gTLD – it requires some development, some adaptation to some specific business models etc.. – but once the investment is done, the cost of selling one or many domains is pretty much the same.
The top level domains should therefore follow the “powerlaw” – ie a logarithmic relationship between the ranking and the volume. Volume should really be turnover, but this turns out to be quite complex to get, as the market is pretty much full of promotions and pricing difference. Some registrars actually bundle some offers, have additional services. Then there are also the premium domain names and the sunrise domains bought by brands – irrelevant of the question wether they recorded to dot brand or not.
The “log log” graph of the repartition of domains per volume is quite interesting.