I'm assuming its to demonstrate there's plenty of room to accommodate a Christmas dinner and bloat
I find this too when I'm trying stuff on, I always think I look huge and the item doesn't flatter me like it does on models or influencers but I've noticed the further back from the mirror you stand with your camera on, the smaller you look. Most of the clothes look completely different when she turns the camera round to forward facing to sometimes show detail in the clothes (pattern or material style etc) .. she's always miles away from her mirror when she films giving the illusion she's smaller