I know comics have done similar things. Ant-Man and the Wasp can switch between who's big and little. Atom had a fling with Giganta in one run (haven't read that run yet personally). But here's the dynamic I'm looking for:

Giant lady grows to be really, really big. That's her power. That's her only power. When she's not a giant, she's an ordinary girl in a costume. Inherently, she's a little insecure that she's gonna be caught in a situation where she can't grow, and she'll be useless. She likes feeling big and strong, and looking down on the world - not in a narcissistic way, just generally looking at the world from that viewpoint. She is initially attracted to the male lead of the story because she can still feel huge around him without growing giant. So she can ask him to be little and cute while she's in a shirt and slacks in her apartment.

On the flip side, the guy often feels a little trivialized because of his small stature. He can grow big of course, but people usually just view him as the stealth guy. The guy who crawls into tiny vents and keyholes. He can take down big threats just fine on his own, but not nearly as flashy. And when people actually have to interact with him while he's tiny... Oof. They rarely treat him the way he wants. Either they're too rough, treating him like a ragdoll; or they're too timid, treating him like glass; or they just ignore his existence until he's relevant to their needs. So he ends up with the lady who can grow huge because she finds that sweet spot. She knows the exact amount of pressure or leniency to apply when handling tiny people because she does it every single day. So she's the only person he feels truly comfortable being tiny sized around.

It's a little sappy and a little cliche, but that's just what's on the brain at the moment.