Genie made herself invisible and entered Donnie's mind to unlock his deepest desire. When she found it, it wasn't what she was expecting.
Donnie was a macrophile; He had a desire for giant women.
"So, he likes big girls, eh?" she mused to herself as she left his brain. "I'll have him under my thumb in more ways than one!"
Genie pondered on what to do. She could grow bigger, but that would attract the attention of everyone else in the town. She could also shrink Donnie, but that would put her beloved in danger of being stepped on and attacked by larger animals.