Honestly, don't do it. Set aside your fetish for a hot minute. Put it aside on a stool, now sit on down here and let me just put some things out here for you that aren't apparent for the age that you're at.
I'm currently 9 years older than you, and while you are an adult, I think it's helpful to gain some outside perspective here. In all of this post, you mentioned your weight and you gaining and you both delving into that lifestyle. That's fine and all, however here is the big thing you have to think about. One. you are still 21. Barely an adult. You still have so much time to go out and have fun and experience your life. Secondly, she has a child. A child, which, eventually, if you stay with her long enough, WILL come to see you potentially as a parental figure, which then means a future role model. This will also mean being actively involved in said child's life, and possibly interacting with the child's step parent. This also means that you will also, more than likely, be helping to cover expenses for a 40 year old woman and her child. This also will mean that you will take on the role of being a father to this child. Moving to be with her is one thing when she's single. Moving to be with her while she has a child is completely different as that is a dependent you have to take into consideration.
You, at age 21, are very much focused on the fetish aspect to where you're focusing not so much on the realistic implications of said move.
Another thing I'm going to ask here is moreso geared into why she's asking and wanting to be with someone so much younger than her? I would be willing to bet that both of your priorities in what you want out of life, while sexually compatible, are probably not as compatible. Your ideals and wants will go through a large transformation once you get past your "party years" and once again once you hit about mid to late 20's. Do not confuse Love with sexual attraction - these things are not mutually exclusive. From how you've worded this, it seems as though she's invested in you in the sense that she's trying to control you, which she has in some way, as you mentioned that "you would wait for her to ask you how big you've gotten." That right, to me at least, there is a bit of a red flag. Because tbh, if that's the only reason she was messaging you, that is not at all a relationship that shows she cares for you in a way that you will benefit by being in a full on relationship would be. If she's messaging you only when it's convenient for her, that's also not a fulfilling relationship, but rather one that's conditional. If that's okay with you, then that's okay. But as a side note to that, If you start losing weight, I would be willing to think that her attraction towards you might change as well.
My advice: Don't think of uprooting your whole life just to satisfy your sexual urges and fetishes. Attraction based through fetishes are conditional, and that's not a good basis for a lasting relationship. If she cares about you, and you her, you'd be doing some really large discussions about your lives, what you want from them outside of sex and fetishes, and if she would value your autonomy if you decided you were to ever decide to stop gaining or lose the weight. If gaining weight is the only way to please her and make her happy, and not the merits of who you are as a person off and on the scale, it will not be a fulfilling relationship for either of you and may wind up with you resenting that you uprooted your life for her.
I know a lot of that was very real talk, but there is a lot more risk in this situation because of her having a child and because it seems as though a LOT of your interactions are based on your shared fetish. I'd rather give you things to think about and take into great consideration before making a choice like this, especially for being younger without as much experience in life under your belt.
But ultimately the decision is your own, but do think through this carefully. Because you aren't just affecting her life or your own, you're affecting her child's too.