I am overweight.

我很胖。

I weigh 20kg.

我的体重是20公斤。

Let's measure your weight.

我们来量一下你的体重。

Step on the scale.

站到体重机上。

You've lost / gained weight.

你的体重增加/减少了。

It looks like you've gained weight.

看起来你好像增重了。

You weigh a lot / a little.

你很/不怎么重。

You look good because you're slim.

因为你很苗条，看起来不错。

You are pleasingly plump.

你胖得很好看。