a woman is scorned from the witch society, and warned not to practice magic, because she's really "bad" at it (her spells always come out wrong)
years later, she's married, and living among normals, always trying (and failing) to get the hang of her magic... causing herself and those around her to appear klutzy
her husband finally catches her, and she has to explain the whole disgrace... which with a little "bad magic" proof, he finally accepts... in the end, he agrees she's really bad at it, and she should avoid using magic... however the "cat is out of the bag"
because he knows, she's more open about "trying" to use magic in little ways in their life. which almost always turn out wrong.
when alone, she spends lots of time practicing making things bigger and smaller (fruit, dolls, etc). whenever she tries to make something smaller, it gets super-tiny.. but eventually she figures out how to just undo the previous spell. trouble is, doing it twice just undoes the undo...
on their third anniversary, she sets up a sexy night for him.. which goes so horribly wrong (and right)
...starts it off with some funny magic. he stops her, and insists on no magic. she gets upset, and talks about how she's been practicing all year. she promises she can do this safely. She demonstrates making an inconsequential object a little bigger, then normal, then much bigger. ("see. i promise honey, this is the most basic spell. plus, I can always undo the last change. nothing can go wrong." she holds two apples in front of her tits "besides, I think it'll be fun to make some things bigger, don't you?" (then she recites the much bigger growth spell, and the apples nearly double in size) This works without a hitch, and gets him kinda excited, and he starts talking about her making her tits bigger, which restarts the sexy mood.
She gets in close, unbuttons his pants, and gets his member out sorta quickly. She notes that "you're not the only one who's getting a bigger toy to play with", she recites the minor growth spell, and his cock gets just a little bigger -- which he reacts to both because of the sensation, and because it actually worked. She says she can of course make him a little bigger still, but he should sit with it for a while, because she can only un-do one change. She talks for a minute about his new size, how of course he was a good size before but now he's perfect, not too big, not too small. How excited she is to get him inside her, "but first, one more spell."
She starts fondling herself, "a little bigger is probably enough." (she recites the little bigger spell) She puffs just a tiny bit in her top, hardly noticeable. (she plays dumb, like her tits are now huge. reacts to the disappointed look. then starts laughing, of course she's kidding. she makes a big show of how he should get ready, referencing the double-size apples, how he probably wants them really big eh? then she does the little-bigger-spell again. he gets a sour look, and she laughs. tells him she's just playing. She can only undo the last one, so before she gets big knockers for him to play with, she wanted to get a little for herself. She poses, and he admits she does look substantially sexier. (she confirms he's okay with his wife keeping the new look, she can undo now if it's too much) He encourages her, and says she could even go a little further -- for everyday that is. She rhetorically checks if he's sure? "Really?? Are you sure?" (she does the little bigger spell) While she's not super-busty, she does look super-sexy, with her top now looking a size too small. "Ohh baby, I love them." She starts checking out her tits. "You're going to have some serious eye candy at the next corporate party, once I get some new clothes of course. Now, I think I've teased you long enough. Time for some bigguns to play with, but just for tonight, okay?" She waved her finger out at him. As he nodded, she was already getting into a dramatic pose, sitting up on her knees, with her legs suggestively spread, her hands cupping up against her tits. "Now, tell me you love my magic."
"I love you honey, and I love your magic too." She smiled. She looked so sexy. Too sexy. In fact, sitting up, she didn't look like she just got a little bustier. She looked taller, bigger. Her top was straining into her sides, and even her bottoms looked kinda tight all of a sudden. "Uh, honey?" He took a sideways look at her, and realized it wasn't an accident, she wasn't just bigger up top, she was clearly bigger all over. "Honey, wait."
"No cold feet now, sugar." (she recites the bigger growth spell) Her lips crack a sinister crinkle as she watches her tits erupt out of her top, pushing out the top of her bra until it looks like a demi-cup. "How do you like them apples, honey?" Even as he stares at her impressive new cleavage, he can tell that just like before, it's not just her breasts getting bigger, it's her whole body. A moment later, he looks up from her tits and finds her looking down from above with a concerned look. "Ohh no! Honey, you can't be getting smaller. That's a totally different spell, I swear!" He knew he wasn't getting smaller, she was getting bigger. However, before he could say so, they were both distracted by a sharp popping sound. The clasps of her top had finally given way, and as her titanic tits rose up past his eyeline, he could see it was now just an awkwardly small piece of fabric dangling against her super-sized body. "Ohh no, what have I done?"
(his body observation.. how amazingly turned on he is. ) "honey, it's kinda hot"
"What? That I turned myself into an oversize sex nymphette? You would say that. Wait! Honey, I can undo the last spell!" (she quickly voices the undo-spell, and shrinks back down to semi-normal size) "I'm sorry, honey. I should have known, I always mess up magic." (she goes on about how she was just trying to spice things up in bed. She tries to get the evening back on track, mentioning that "at least she didn't mess things up too bad"
... he badgers and badgers her until he turns the conversation back to making her huge again. She's surprised, but eventually agrees she can do it, and does another undo.. she's quickly nearly double her normal size (10' tall). Then she proceeds to make fun of him for wanting her so big, while also letting him 'play' with her. She repeatedly "checks", "are you sure? if you want I can go back to normal. okay."
...eventually she's getting "into it", playfully pushing him around and dominating him with her size and strength -- with an undercurrent of "she doesn't get it". "wow, you like this? this is a side of you i never knew about it. if I knew I could get you this hot for me just by screwing up a size-change spell I would have done it a long time ago."
From here on out.. she uses her "undo" quite often... (though maybe it should change the size of her clothes too?)... reverting to her giant-form (or threatening it) in situations and places that are increasingly risky. (in the kitchen- right when he gets home from work, around the corner at a friend's house, on the way to the bathroom)