A big secret to the Olympics bids is that most people never read the official IOC candidature application packages to get the facts and scope of an Olympic bid. People frequently JTNC (Jump To Negative Conclusions) that there "must be" a minimum city population as a requirement, when in reality City size and population are clearly not listed as an IOC spec or minimum requirement for city population.
The the first occurrence of the word “Population” in the official IOC bid package occurs on page 86 of the official 2020 IOC candidature application, which asks about horse populations that had infectious diseases in the past 5 years.
The word “Population” shows up only twice (two times) in the 2020 Official IOC bid document. See for yourself by looking in the 2020 Candidature document, and do a search with Adobe (Use Control-F). http://www.olympic.org/Documents/Host_city_elections/2020_CAP.pdf
The second, and final occurence of the word “Population” occurs on Page 97, and simply asks for the population statistics.
Look over the IOC's 2020 Candidature Acceptance Procedure in this websites References section. The Official IOC application is mostly asking about venues sizes, and venue proximity to the Host city.
The Host City size/population might be an indirect measure or its capability to host the games, however, the true metrics used by the IOC to measure a City are: Dorm beds, Venue locations, Athlete village locations, Hotel rooms, and transportation infrastructure for the visitors.