The only fool-proof way of reducing the number of units in a game is make them hard to get ... You can have huge maps with little units if you make them hard to get ( say , the equivalent of getting a rifleman in ancient ages cities in Civ IV ) or a lot of units in small maps if you make them ridiculously cheap, so any reasoning based on the ( desired or not ) number of units is atleast shaky.
all right lets start from here:
1) assuming in any unit rushing mechanic the rush "cost" is proportional to the unit's cost, the number of units is inversely proportional to their cost
2) assuming constant resource density, a larger map will have more resources, than a smaller map
3) a larger map has more tiles, than a smaller map
do we agree so far? i sure hope so!
so let's more on:
since in civ5 units will be capped by their required resource(s), from
2) we get that on a larger map, the resource cap will be higher, meaning that generally a larger map will have more units.
point
3) means that a larger map will generally have more cities. assuming the existence of resourceless unit types, this also means more units.
so what?
from firaxis's point of view, more units is undesirable
My gut feeling about the size of the maps comes from other considerations ( not all from gameplay, though : one of the biggest and more consistent pet peeves about Civ IV was the small size of the maps compared with previous versions of the game ... )
shrinking map sizes in civ4 probably had a lot to do with the 3d stuff. the 3d stuff is in civ5 also, so i would not expect a major increase in map sizes.
and second! does firaxis actually expect people to maintain 20-30 tile fronts and love it!?