Spoop Skerry:
Good old fashion cursed gift. Something like, a spurned sister, cousin, coworker, etc., gives protag a giant ugly sweater for Christmas. Protag grows fat enough to outgrow the sweater, extended family commenting on appetite and weight as they blow up.
Could be a reality shifting tale also, all the family oblivious to the new weight while protag and gift giver being aware.
I have a story I posted here, "Holiday Dreams". I guy buys some lingerie from a off the wall site, saying the items are from Mrs. Claus. He puts them under the tree, with an open me first note. The fun begins.