If you are sure you dont want to use BigCity then you can delete these in file manager or total commander etc not in UE as it will take ages.
Content/ExternalActors/Map/Big_City_LVL @ 8.25Gb
Content/City/Big_City @ 350mb
Wouldnt touch anything else really, but if you are packaging your Small_City_LVL why are you bothered about BigCity , that has no effect on your packaged build, perhaps you have some settings not set correctly