变胖用英语可以表达为 "gain weight" 或 "put on weight". "Gain" 这个词表示"获得"，"put on" 则表示"增加"，两者都可以用来描述体重增加的情况。例句如下：

I gained a lot of weight after I stopped exercising.

She put on weight after she had her baby.

"Gain weight" 和 "put on weight" 这两种表达都是源自英语中的动词 "gain" 和 "put on"。"Gain" 这个词原意是"获得"，指获得某种东西，例如：

She gained a lot of knowledge from reading books.

He gained confidence from speaking in public.

"Put on" 这个词原意是"穿上"，指穿上某件衣服或配饰，例如：

She put on a coat before she went outside.

He put on a hat to protect his head from the sun.

当 "gain" 和 "put on" 用来描述体重增加时，它们也可以表示"变胖"的意思。例如：

I gained weight because I ate too much junk food.

She put on weight because she didn't exercise enough.

总之，"gain weight" 和 "put on weight" 都是指体重增加，可以用来表达"变胖"的意思。