Her loli older sisters called her huge, so maybe she's not only tall but also thicc.
Then again, they were also outright bullying her for growing at all, since she was the only one capable of it among them and in so doing deviated from the "ideal beauty" the sisters represented.
Going by her older Gorgon form which exaggerates/advances her figure, Medusa is meant to be shapely, though. Not too THICC, but with a blend of being toned with long limbs and enticing curves. And big boobs.
Medusa has always been decently shapely, with noticeable breasts, hips and thighs. I'd say this image is pretty accurate, even. Furthermore, her curves add to that 'big sister' look she's got in her casual wear, and she serves as one such figure to Sakura.
