Let’s say she’s got this super power that makes any amount of muscle strength above the norm a ridiculous magnitude more powerful, complete with toughness to go with it. She could bench press a fully loaded big rig, uproot any tree she can get her arms (and boobs) around and rip that tree in two with a little effort when she’s at her peak. She’s also got a metabolism that builds muscle extremely easy, but she seriously struggles to lose weight, hence only a truly dedicated partner could turn her into the hourglass figured version, but she’s always going to be ‘big’; at her slimmest, she has a waist/core as wide as a man, but hips and a bust big enough that she sometimes struggles to get through doors without angling herself right. She’s a 6’3 to 6’10 (your choice) girl and her frame was designed to help her super strength and toughness.
Now, let’s say back in high school, she was into fitness and weight lifting, then awakened to this power and the other kids called her a freak, and even realised she didn’t stand up for herself until she literally punched someone through a wall, badly injuring them and mentally traumatised herself for losing control, and that cemented that she’s a freak in her own mind.
Deep down, she still wants to be fit, strong and muscular again, but alone, without that assurance you’ll not be scared of her and you find her strength and physique sexy, she can’t drown those doubts out and she’ll let her fitness fade, since it’s easier to function when she doesn’t have to always be careful and just being fat is easier to deal with.
I guess as a caveat you could say you need to be motivational and extra supportive for a good ten years or so before she finally breaks out of the trauma and no longer needs you to keep pushing her. As a bonus, her gratitude would motivate her to even maintain her extremely difficult to achieve ‘slim’, but curvy figure if you preferred her that way.