DaviddesJ
Deity
For the no Slavery case, they don't necessarily have "surplus food". Surplus food doesn't depend so much on city size. It depends on whether you have surplus food, period. In fact, a larger city will tend to have less surplus food, because the governor will work some mines automatically; heck, a human will work some mines. Production is a Good Thing(tm).
Everyone playing the same map has the same amount of food. Everyone will also have cities of almost the same size (Slavery will mean only -1 to happiness). The difference is that with frequent use of Slavery you'll rarely be at the case where you have a lot more food than you can use. If you never use Slavery, then your cities will often reach their happiness limit where you have extra food but you don't want them to grow any more.
For Slavery, IMO building a worker is a great thing to do while you're waiting for the city to recover from the whip happy penalty.
More usually you are just regrowing the several citizens that you used up when rushing. Sometimes, in cities with a lot of food, you can do that faster than the unhappiness timer for rushing again. But you're still spending much less time at this limit than if you never rushed at all.