bad是坏事的意思，而fat则是肥胖的意思，区别可以从以下例子看出：Overweight is not a good thing, as long as you are not too fat, otherwise it is bad for our health. We have to exercise to maintain our health. 超重不是什么好事，只要你不是太肥胖，否则对我们的健康是坏事。

我们必须锻炼以保持健康。