you could try increasing the swap partition (UN*X) / paging file (NT) size. This will prevent memory allocation failures. however, this is bought at the price of a severe performance penalty – in particular if the device hosting the swap / paging file is a mechanical HDD and not a SSD drive.
as for the question whether or not there is a memory leak present, it would have to be investigated how memory consumption evolves during a certain period of time. a tool that would come in quite handy to do this in a graphical way would be sysinternals process explorer. however, it is true that increasing memory consumption may very well be caused e.g. by the increasing amount of population / vehicules, etc.
so to somewhat harden the evidence there is a memory leakage, you might e.g. try to run a game session for some time (monitoring memory consumption. then re-loading the same save-game you started with and comparing memory consumption (preferrably, this ought to be repeated several times).
cheers,
Christian