Instead of simply stating a size, describe the movement or how clothes sit. For example, mention the "strain on a button-down shirt" or the "relief of unfastening a bra at the end of the day".
Full-chested, well-endowed, heavy, broad-shouldered. Classic/Formal: Buxom, voluptuous, statuesque, Junoesque. Casual/Modern: Curvy, stacked, built, curvaceous. Narrative Perspectives
Writing about characters with a "mega big busty" physique can serve different purposes, from focusing on body image and physical challenges to stylized genre fiction like romance or fantasy. Descriptive Vocabulary