Unlike any other day, Undyne spends her day either fasting or bulking up while fasting. Undyne being a belly spouse is like living inside the belly of a titan that's actively growing... because that's what she intends to do. She takes this very seriously, not only because it's her responsibility to take care of you within her gut, but also the thrill of getting bigger and bigger until she's like a impenetrable fortress. Not to mention, it gives you more room inside the stomach. Right now, she finished her third buffet, doing a belch that might as well shatter a good amount of glass within her radius.