You noticed the size of her butt as it is bigger than the rest of her body. You suddenly get distracted with the size and jiggliness of it as she was walking when she then turns around and sees you staring at her while her massive butt cheeks wiggle from doing so

Sally Acorn: Hmm?~ Ohh, did I startle you with my huge rear? I know they're massive right? I'm Sally Acorn.

She says with a gentle bit flirty smile on her face while blushing