There's a reason Amy was able to already be fairly plump by 5th grade, and anyone who saw her out with her mom would understand. Amy herself just thought of her mom as "kind of big" for two reasons:
1. She had grown up with her mom all her life, which normalized fat
2. She constantly read weight gain fiction of absolutely massive blimps blowing up to totally unrealistic weights
So from Amy's perspective cultivated within weight gain sites, her mom was a pretty normal American woman. The reality of the matter was that Amy's mom, Sarah, was a 350 pound lazy pig who loved snacking and let her daughter eat whatever garbage she wanted both because she lacked perspective on a healthy diet for a child, and because she didn't want to be a hypocrite when talking to Amy about diets and weights. In fact, the sweetened peanut butter Amy had been eating last night was some of the healthiest food available in the house aside from fruit, and she was having a cherry coke because there was no drink in the house aside from soda, unless she wanted to get a cup of water from the unfiltered tap. Amy stuck to stuff like the peanut butter because she only got the sense of what a girl her age "should" be eating from the kids around her at school. She was a "good" girl, and while she indulged her hunger whenever she felt the urge, the only time she would have overt junkfood is when her mom would bring it to her... which was not infrequent. The result was that Amy, an average-height girl at 4'6" and 10 years old, already weighed 130 pounds and was also considered seriously overweight for her age, but this left her looking nowhere near as rotund as her mother... yet.
So when Amy came into the kitchen where her mom was making herself a usual snack of four nutella-banana sandwiches, she genuinely was quite nervous. Although fat had been normalized for her, she knew she wasn't supposed to want to get fatter, and her goal wasn't to look like her mom... it was to outgrow her mom by quite a bit.
So she approached the topic cautiously....
"Mom... can I talk to you for a bit?"
This was an unusual carefulness, so Sarah took it seriously. After licking the nutella off her knife, and shuddering. Damn that gooey chocolate spread never got tiring.
"Yeah, what is it honey?"
"I... I'm already kind of bigger than the other kids at school, right?"
"Oh, honey... you look wonderful! You don't need to think about things like that! You're a growing girl and you should just focus on being happy. Are you feeling okay? I mean, if you really want to insist on a diet then I--"
"NO!! I mean... sorry... no, that's... I was just thinking... I like being big... I wanna be a lot bigger...."
Sarah paused. She liked being big too. She loved indulging in her every desire. Most of the house was filled with snack food, and she handed some off to Amy pretty often but she noticed Amy didn't take much more of her own accord, so this was... a genuinely unexpected turn. But she genuinely didn't mind, and more importantly didn't wanna be a hypocrite.
"Oh! Uhm, that's okay honey! How big were you thinking? You know you can eat anything in this house, right? I buy it for both of us, not just me...."
"Uhm... I was thinking... maybe a thousand pounds would be my goal weight? For now... I kinda wanna be as big as possible but I might wanna stop before immobility.... So can I just keep eating to try to get bigger, mommy?"
Sarah... was confused. She had never read weight gain literature, she just loved indulging. She had no idea where a thousand pounds came from. She was starting to get worried that her kid wasn't very bright, as she was 100% sure that immobility would happen before a thousand pounds. But she also figured it was such an unattainable goal that she'd just try to be supportive for now.
"Uhm... wow, that'd be about three times as big as mommy! But like I said, you can eat whatever you want... I can also buy even more junkfood if it'll be useful to help...." Sarah was actually starting to drool when saying this part, having a kid who wanted to get fat might actually be a dream come true. "I can fill the house with more desserts... snack cakes, pies, brownies, fudge... is that what you want honey? I can also bring you more snacks while you watch TV and use your computer, if you like!"
Amy was really happy with how well this was going.
"Yeah!! That... that'd be great!! Thanks mommy! I really want to sit around all day and stuff my face constantly!!"
Her dream really was going to come true. She made the right choice talking to her "kind of big" mom, after all!
From that day on....