One of the biggest issues with trading intelligence for anything is that you need to be very aware of exactly how much intelligence you have to spare.
Unfortunately, those who are willing to bargain away their intelligence are the most likely to vastly overestimate how much they have.
She had everything she needed to complete the aesthetic that burned in her mind, that of the Big Titty Goth GF. Pale skin stood bright against dark hair. Shades of red and purple and black dominated her wardrobe. Dusty tomes (and a few vampire romance novels) filled the bookshelves of her bedroom. Her corsets were not magic, however, and they couldn't conjure boobs where there simply weren't any to be had, no matter how tightly she tied them. That's where her plan came into play. While her corsets weren't magic, she reasoned, she could simply employ actual magic to get the curves she wanted! A quick deal with an ancient goddess, one who would surely support her in her quest for divine femininity, and BAM! The aforementioned big titties would blossom on her chest and complete her look. All she'd have to do is trade away a bit of her intelligence, but the simple flawlessness of her plan was enough to convince Dawn that she had it in spades.
She made only two mistakes. The first was that, no, she didn't have that much to spare. The second, which is related to the first, is that she grossly underestimated just how much intelligence it would take to get the size she wanted.
With the candles lit and the incantation spoken, Dawn was delighted by the ethereal wind that whipped around her. A moan trumpeted from her throat as her hands flew to her chest. She couldn't believe it! She kneaded her breasts as they swelled and engorged in her hands, tightening her shirt as they pushed against the fabric. Her smile never faltered as her breasts expanded- er, her boobs became fuller and heavier- er, her tits grew- um, her titties were, like, totally getting bigger!
She gasped and giggled at their immense size, bouncing them and wobbling them as they overflowed her hands. They were soooo big! And, like, super sensitive! Dawn moaned, holding nothing back, as she played with her shirt-straining, eye-catching, sloshing, jiggling assets. The thought to try and undo some of the changes didn't even pass through her empty head and, even if it had, she wouldn't have been able to read the words in them anymore. That didn't bother her, though. She was too dumb to be bothered by it, especially when she was so pleased with her new bouncy boobies.
Even though she now had the big tits of her dreams, Dawn didn't quite fit the image of the Big Titty Goth GF. She'd find much more happiness as Danni, the Big Titty Goth Bimbo!