I would bet that the city unhappiness penalty is dependent upon map size for the reason that each city is more important on smaller maps.
You are also making yourself more vulnerable to attack; and depending on how military supply works someone with the same population - but more cities - is probably able to field a larger army due to increased number of tiles (and thus resources) available to them (without relying on city-states) and a larger supply threshold.
Now, the presence of Golden Ages and their tie-in with happiness will help offset this production disadvantage somewhat.
My gut, too, says that 50% on population is too much. 25% on population and, depending on whether it is map-size dependent, either +100% or a static +2 or +3 unhappy per city would put break-even around 8-12 population points. If the break-even point is too low then while the advantage is merely good in the early game it becomes super-great toward the modern when, for any given map size, the ratio of population/cities is going to be high for everyone.
I find the name a mis-nomer as well since growth (a function of surplus food IF you are happy) is not really being affected (aside from avoiding unhappy) but rather population capacity (a function of both happy and food) is.