Anime / Manga - 着ぶくれするタイプ (The type that wears extra clothes)

I think the implication is she was already buff, but her co-worker assumed she was fat because of her bulky clothes, and once they changed at the gym the co-worker was like "holy shit, I did not expect that."

Thicc Buff….THICC BUFF…..THICC BUFF!!!!!