It doesn't, only losing cities does.
The cities are probably more defended because you spend time getting your artillery into place. This allows them to draft and move defenses to the city because they know you're about to attack it. Faster conquest = less defense. Also I said 2 infantry, not 2 defenders. Usually there's 2 infantry, a pikeman or musketman, and a spearman or something like that. The tanks just roll over the last 2 though.
It would be nice if size 20 cities were only defended by 2 regular infantry, but that sure never happens in my games. If a city is size 20, it's almost always defended by at least 3 veteran infantry as well as 1-2 regulars or conscripts. With promotions, etc, it would take at least 15-20 tanks to kill all the infantry off in one turn.
The cities are probably more defended because you spend time getting your artillery into place. This allows them to draft and move defenses to the city because they know you're about to attack it. Faster conquest = less defense. Also I said 2 infantry, not 2 defenders. Usually there's 2 infantry, a pikeman or musketman, and a spearman or something like that. The tanks just roll over the last 2 though.