The advantages are:
- More tiles worked, yielding a larger Hammer stream in the same place, speeding production of any individual output
- Since you have decent population, you can run specialists AND build stuff at the same time
- More Science from your buildings cumulatively acting on raw population Science production
- Higher defense rating, making the city harder to capture
The disadvantages are:
- Massively slower population growth
- Only one buffed city tile. The rest of the tiles are less efficient and require Worker improvements to become comparable.
In practice, small cities win out despite the advantages of large cities. A few buildings are extremely powerful, and you want as many copies of those buildings as possible. Also, Maritime food benefits operate on every city, meaning that lots of cities are the Food maximizer and therefore the population maximizer. The buffed city tiles mean that lots of small cities is also the overall Hammer maximizer.