Interestingly, by removing curruption (Civ 3) and distance-to-city maintainace (Civ 4), there's nothing to prevent large civilizations to snowball in tech.
One thing that I liked about previous civ games (and Alpha Centuri, University race?!) is that a small civ with a single or couple of cities can often out-tech large empires. This is due to the fact that additional, distant cities grow more and more expensive and only marginally improve your research (or even cost more maintainance than they make!), while more cities increase the research cost.
In Civ 5, you get 100% of the research from cities, no matter how distant it is from your capital. Double the side of your empire? Double research! The only downside to overexpanding is that your culture bar would be absurdly high and you'll never get that many social policies, but that hasn't even hindered me in my (single) playthrough of Civ5. Social Policies are just nice little bonuses, it doesn't matter if you don't have them if you are teching at triple the rate of everyone else.
So I think you are falling victim here to China's glorious snowball. They have a large population, they tech faster, they have better units, they take cities, they have a larger population, they tech even faster....
Actually, now that I think about it, I guess every civ game had a snowball, its just that in the past it was more like, you have a high commerce income, you tech faster, you can now build better buildings/wonders, you have a even higher commerce income, you tech even faster....
