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, which is called "Christmas tight".

在圣诞节和新年之前穿着很合身的衣服，在过完节后再穿忽然变紧了，这就叫“圣诞缩水”。

"Christmas tight" happens because there is so much delicious food during the big holidays, and you just cannot help yourselves. Therefore you put on weight in a few days.

发生“圣诞缩水”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。结果几天内体重就增长了不少。

Example:

"I got this new dress shirt for Christmas. I went to wear it with my favorite work pants, but they are a little Christmas tight now."

“我圣诞节收到了这件新衬衫。我把它和我最喜欢的工作裤一起搭着穿，但是这裤子现在有点‘圣诞缩水’了。”