Christmas tight refers to the period of time after Christmas and New Years where the clothes you wore prior to the holidays that fit well, all of a sudden are a little tighter than they should be. 

圣诞节和新年之后，你发现你节前穿正好的衣服，居然有些紧了。这就是“节后缩水”。

Christmas tight happens because there is so much delicious food during the big holidays, and you just cannot help yourselves. 

发生节后缩水是因为在这种盛大的节日里美味的食物太多了，你忍不住狂吃。