Establishing the ground rules early on is key.
"Does my arse look big in this"
"You know you haven't got a big arse, but that skirt/dress/jeans don't flatter you as much as that other skirt/dress/jeans. It's not cut for a bigger figure, it's not you"
And then there's those who prefer the radio silence which follows such statements as "Does my . . . . . ?".