Any warmongering civ is quite good. I still think China is a top tier civ. Once you can take chu-ko-nus and a GG you've got the game in hand. The paper maker is still an excellent building too and it helps when you have lots of puppets.
About this population thing. It's about context and knowing how to win quickly. It becomes clearer why high population cities simply pale in comparison to size 7 or 8 cities once you do.
You see most people think that a game lasts almost the full 550 turns. In reality once you know what you are doing you can end a game usually between 200 - 250 turns. The latest patch slowed this down a little so those numbers may be a touch low. The first 100 or so turns are spent pre-renaissance. The last 100 - 150 are spent afterwards, obviously. Most of the buildings and wonders in the renaissance and afterwards are not necessary to win and will not pay for themselves before the game is over. There are a few exceptions (like the Factory, Solar Plant, or Nuclear Plant).
How do you achieve this? For all but 1 victory condition, the answer is GSes and RAs. To get GSes you need universities, which incidently is one of the most expensive buildings you actually need to build. To get RAs you need gold. Anything that does not produce gold or help generate another GS is useless to you. So temples, windmills, banks, stock exchanges, watermills, colloseums, theatres, museums, etc are all useless to you because they will either cost you gold or will not produce enough gold by the end of the game to sign an additional round of RAs. You also don't need specialists except for scientists so getting a big city to run them is not needed (unless you are aiming for cultural victory).
So you don't want to grow many of your cities any bigger than about size 7 or 8 because after that you need happy buildings which cost gold upkeep.
So basically you start. Expand to 4 cities or so. Build universities. Fill them with scientists. Get the PT and open rationalism. Sign RAs and generate GSes as quickly as you can. RA and bulb your way through Renaissance, Industrial, to Modern. Get the tech(s) you need for your win condition. Win. Babylon and France are probably the best for this kind of play. Siam or Polynesia are good for culture. Domination is anyone with a good early or gunpowder to rifling UU.
Of course you don't have to play this way and I suspect 90% of people do not but I don't like it... it's why I argued against RAs before this patch came out as they are very, very easy to abuse. Even GSes are too easy to abuse.
You're right about the faster games and the consequences on strategy. It supports Martins point.
However, you won't win any game with 4 cities @ size 7-8. Either build more cities, grow or conquer.
More population = more hammers/science/gold. That's a fact.
Saying that happiness and culture buildings are useless because they cost hammers and maintanance is flawed logic, too. They INCREASE gold output because your cities can work more tiles.