"If you see early films of London the streets were just as crowded as they are now but with horse drawn traffic. I suppose, however, that London itself is bigger."
There were 300,000 horses in London c.1900 and there are 2.6million cars registered in London today. In that time the human population has only increase by about a third (Wikipedia says from 6.6E6 to 8.6E6).
I reckon the horses produced more pollution, too.