变胖在英语中可以用 "gain weight" 或 "put on weight" 来表示。这两个短语的意思都是增加体重。

"Gain weight" 来源于动词 "gain"，其本意为 "获得、增加"。例如：

I gained weight over the hopdays because I ate too much. (我在假期吃得太多，所以体重增加了。)
He has gained weight since he started working at the office. (自从他在办公室工作以来，他的体重增加了。)

"Put on weight" 来源于动词短语 "put on"，其本意为 "穿上、装备"。例如：

She has put on weight since she stopped exercising. (自从她停止锻炼以来，她的体重增加了。)
I need to watch my diet or I'll put on weight. (我需要注意饮食，否则我会变胖。)