Frisk, somewhat reluctantly, finally decided to humor their adoptive mother's wishes and nodded, holding out their plate for a slice. Toriel smiled wide and gladly plopped one down on his plate before digging into her own slice. The moment the slimy, snaily goodness of that slice hit Toriel's tongue, an explosion of flavor unlike anything she'd ever experienced before washed over her, overwhelming her senses to the point that she could only moan in absolute bliss at the taste of it. She began to dig into her slice of pie as though she were starving, slightly disturbing Frisk as they slowly guided a forkful to their own mouth.
In it went, and Frisk's normally narrowed eyes went wide. The texture was, to be frank, disturbing, almost enough to make Frisk not want to swallow. But the taste... it was so good that Frisk managed to push through the initial disgust and send it down before going back for another bite. Their progress through the slice was slower than Toriel's though, and by the time they had finished the delicious yet slimy treat, their mother had already devoured the entire rest of the pie in her frenzy. Frisk, deprived of the chance to have seconds, huffed silently at Toriel with a look of frustration, causing the boss monster to blush as she came down from her gluttonous frenzy.
"Oh, dear... I'm terribly sorry, my child, I've no idea what came over me! Tell you what, why don't I make another tomorrow, and then I promise that I'll let you have as much of it as you want!" Frisk seemed to accept this and nodded, still huffing at having to wait.
With that, dinner progressed as normal for Frisk as Toriel, full from the pie, began to clean up. As she did, though, she felt... odd. There was a strange warmth in her stomach, along with a feeling... as though she could feel herself digesting her meal. The slight bulge the pie had added to her belly was rapidly dwindling, and as it did the warmth spread to her chest and rear. Both were already fairly impressive on her tall, thick figure. But, as she carried dishes to the kitchen, she could swear they felt more... cumbersome than usual. Her robe seemed tighter in the chest and hips, and she swore she could feel it rising on her. As she began to bump into things with them, she looked down and gasped to find them swelling out before her eyes, albeit rather gradually, only just fast enough to see.
Toriel's thoughts raced for explanation before the only difference from her normal routine came to mind. The flavoring she'd added! It must have caused this! "Oh, I should have known this was some sort of practical joke! Well, I don't find it funny!" She stomped her foot, causing her newly enlarged breasts to jiggle under her tightening robes. Before, she was proportionally about a D-cup. Now, she was reaching and passing F-cup. Realizing this, she couldn't help but blush a little... as upset as she was, she couldn't deny there was something... pleasant about this...
She reached up to grope herself, moaning softly from the sensation before stopping as she realized something. Frisk had eaten the pie too... less than she had, but still... she rushed out to see what was happening to her child, crying out "Frisk!" in a panic as she saw...